
Class. 
Book. 






Copyright^ .. 



CflEffilGHT DKFOSm 



THE ANALYSIS OF 

RUBBER 



BY 

JOHN B. TUTTLE 




American Chemical Society- 
Monograph Series 



BOOK DEPARTMENT 
The CHEMICAL CATALOG COMPANY, Inc. 

19 EAST 24th STEEET, NEW YORK, U. S. A. 

1922 
Monograph 



Copyright, 1922, By 

The CHEMICAL CATALOG COMPANY, Inc. 

All Rights Reserved 




^ qa 



CI.A6U004 4 



Press of 

J. J. Little & Ives Company 

New York, U. S. A. 

NOV -7 .922 









GENERAL INTRODUCTION 

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3 



4 GENERAL INTRODUCTION 

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GENERAL INTRODUCTION 5 

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PREFACE 

The tendency of the industry is towards simplification of 
methods and materials. The readjustment of conditions to the 
basis of an adequate supply of crude rubber — a condition which 
did not obtain twenty years ago — has by the operation of natural 
economic laws eliminated from general use many pigments, rub- 
ber substitutes, and low grades of rubber. These are not likely to 
return, and we may dismiss them from consideration, confining 
ourselves to the materials found in the every day life of the 
industry as it exists today. 

In any work on analysis, and especially on industrial sub- 
stances, it is impossible to avoid the presentation of the subject 
from a very personal point of view. Many methods, and modifi- 
cations of methods, are written on a single phase of the analysis, 
with a great variety of purposes back of them. In the analysis 
of rubber, methods have been published because they were shorter 
than existing ones; some used less expensive materials, or more 
simple equipment; and some because they really were an improve- 
ment. Few of these methods were thoroughly developed before 
publication; the user must discover for himself the limits of error 
and applicability. It is usually safer to hold fast to such methods 
as have stood the test of time, and whenever there may be any 
methods for any part of a rubber analysis which are not included 
herewith, it is because data are lacking as to their ability to 
accomplish the desired purpose. The omission does not imply 
lack of merit, but merely that sufficient experimental evidence is 
not yet forthcoming to warrant an unqualified approval. 

Primarily, this monograph is addressed to the chemists in the 
consumers' laboratories, and to those who, without any previous 
experience in the technology or analysis of rubber, may be called 
upon to deal with a problem in which the composition of rubber 
may play a more or less important part. Nevertheless, it is the 
author's hope that it may not come amiss to those colleagues 
comprizing the technical staff of the laboratories of the manu- 
facturing plants, who may find it desirable to study a competi- 

7 



8 PREFACE 

tor's product, or who may be required to produce materials to 
accord with the consumers' specifications. 

In view of the probability of this monograph reaching chemists 
of limited experience in the technology of rubber, Appendix A, 
on the methods of preparation of rubber compounds, and Ap- 
pendix B, on the physical testing of rubber, have been added. 
These appendices are necessarily elemental in character, but they 
may serve as connecting links between these subjects, and the 
chemistry of the analysis of rubber. 

J. B. T. 



TABLE OF CONTENTS 

PAGE 

Preface 7 

CHAPTER 

I The Purpose of Rubber Analysis; What Is Rub- 
ber; the Need for Chemical Analysis of 
Rubber 11 

II The Composition of Crude Rubber; Constitu- 
ents of Crude Rubber Other Than the 
Rubber Hydrocarbons 16 

III The Preparation of Rubber Compounds; Crude 

Rubber; Reclaimed Rubber; Oil Substitutes; 
Mineral Rubber ; Mineral Hydrocarbons ; Oils 
and Waxes; Vulcanizing Materials; Organic 
Accelerators; Inorganic Accelerators; Inor- 
ganic Fillers 26 

IV Theory of Vulcanization; Cold Vulcanization; 

Vulcanization With Mixed Gases; Ostro- 
muislenskii's Theories of Vulcanization . 57 

V Sampling 64 

VI Extractions; Acetone Extract; Chloroform Ex- 
tract; Alcoholic Potash Extract; Analysis 
of the Acetone Extract 68 

VII The Determination of Rubber; the Tetrabro- 
mide Method; the Nitrosite Method; the 
Indirect Methods; Difference Methods . . 76 

VIII Sulfur Determinations; Total Sulfur; Free 
Sulfur; Sulfur of Vulcanization; Sulfur in 
Fillers 84 

IX Detection of Organic Accelerators .... 94 



10 TABLE OF CONTENTS 

CHAPTER PAGE 

X Mineral Analysis; Special Determinations; 

Specific Gravity 97 

XI Micro-sectioning and Microphotography . . . 114 

XII Calculation to Approximate Formulas . . . 117 

Bibliography 121 

Appendix A Preparation of Materials for Rubber 

Manufacture 139 

" B Physical Tests 143 

" C Table of Specific Gravities 149 

Index 153 



THE ANALYSIS OF RUBBER 

■ 

Chapter I. 

The Purpose of Rubber Analysis. 

The growth of the rubber industry has been tremendous, espe- 
cially, so far as volume is concerned, since the advent of the 
pneumatic tire. More than any one other cause, the resiliency 
afforded by the pneumatic bicycle tire was responsible for the 
wide spread popularity of the bicycle, and the rubber automobile 
tire has played an equally, if not more important role in the 
development of motor driven vehicles. In the production of the 
various rubber articles, besides the essential rubber and sulfur 
which make up the vulcanized rubber, we need a vast volume of 
pigments or fillers, because by their use we may modify the 
properties of the vulcanized rubber so as to attain a degree of 
service which would otherwise be impossible. We must not, 
therefore, look upon these added substances as adulterants, or 
even mere diluents, but as integral parts of the whole, for by 
their service they have earned the right to due consideration. 
Some, it is true, are largely valuable, owing to the fact that they 
lower the cost of the products in which they are included. The 
rubber industry makes big demands upon the producers of raw 
materials, such as zinc oxide and sulfide, lead compounds, carbon 
black, magnesium oxide, and talc, and were we forced to depend 
upon these pigments alone, the costs would soon rise to prohibi- 
tive heights, with concomitant injury not merely to the rubber 
industry, but to others as well, such as paints and inks, which 
depend for their existence upon an adequate supply of these same 
pigments. We are thus doubly obliged to seek far and wide for 
new materials which will accomplish one of two things: produce 
the same or even better quality at a lower cost, or a better 
quality at the same cost. 

In every line, there is a more or less clearly defined standard 

11 



12 THE ANALYSIS OF RUBBER 

of service, and the future of the industry is quite definitely tied 
up with the results obtained in the present by the attainment of 
that standard. In order to use correctly any material, it is 
important that we know the degree of purity of the commercial 
grades, the influence of possible impurities upon the quality of the 
products, and, most important of all, the degree of uniformity 
obtainable from one period to another. These data can be 
secured only through careful and persistent testing of the raw and 
finished materials. 

It is not our purpose here to undertake a description of the 
functions and usage of the various materials to be mentioned 
later, but merely to discuss them from the point of view of their 
chemical properties. For the various other phases of the subject, 
the reader is referred to the bibliography which is included here- 
with. 

What Is "Rubber"? 

Probably few words in general usage are applied as generally 
as the word rubber. Strictly speaking, the word belongs to the 
polyterpene having the formula (C 10 H 16 ) X . We know, however, 
that there is an homologous series of these polymerized products 
differing from each other by the constant quantity 2 CH 2 , and 
these products have so many of the qualities peculiar to 
(C 10 H 16 ) X that they too are called rubber. Thus we may say that 
there is a rubber series analagous to the paraffin series, etc. 
The planters who cultivate the plantations call their product rub- 
ber, or crude rubber, although, in addition to the polyterpene, 
there is 2% and upwards of acetone-soluble substances called 
resins; 2 to 6% of nitrogenous substances, akin to the proteins; 
and small amounts of substances possessing the properties of 
catalysts of the vulcanization process. The manufacturer pro- 
duces rubber products, although in addition to the rubber as 
obtained from the plantations, many other substances are added, 
both organic and inorganic, because of certain qualities which 
such additions produce in the finished article. Moreover, chemi- 
cally speaking, we have in the hot vulcanized articles an entirely 
new series of products, viz., the sulfur addition products of the 
polyterpene, a series which passes from the extreme of pure 
rubber at one end to a constant composition of (C 10 H 16 S 2 )x at 
the other end. 



THE PURPOSE OF RUBBER ANALYSIS 13 

In order to avoid confusion, for our own purposes we will use 
the term "rubber" to mean any mixture of (C 10 H 16 ) X (its homo- 
logues are negligible commercially, at present) with any other, 
substances, in either the vulcanized or unvulcanized state. 
"Crude rubber" will apply only to the materials as obtained from 
the rubber trees; and where the polyterpene itself is indicated, 
we will use the term "rubber hydrocarbon." "Rubber compound" 
will be used to indicate the formula of a commercial mixing. 

From an analytical point of view, it is of little consequence 
whether the unit in the molecule of rubber is C 5 H 8 or C 10 H 16 . 
There seems to be a preponderance of evidence in favor of the 
latter; in any event, we do know that while rubber can be 
synthesized from isoprene, CH 2 :C(CH 3 ) .CH:CH 2 , the rubber 
molecule itself contains only one double bond for each group of 
C B H 8 . By vulcanizing rubber with a large excess of sulfur, C. 0. 
Weber obtained a hard rubber corresponding to C 10 H ltt S 2 ) x . 
With bromine, rubber has been found to form (C 10 H 16 Br 4 ) x . 
These experiments have been repeated so many times, that there 
seems to be no necessity for further argument as to the existence 
of the two double bonds. The importance of this fact is seen 
when we realize that this fact is the basis upon which have been 
built the two classes of direct determinations of rubber, the 
tetrabromide, and the nitrosite, both of which will be discussed 
in their proper place. 

The Need for Chemical Analysis of Rubber. 

Any scheme which may be suggested for the analysis of rubber, 
either vulcanized or unvulcanized, must take into consideration 
the fact that there are two groups, with widely differing points 
of view, which are interested in the subject. We have first the 
manufacturers' chemists, who test their own products to deter- 
mine what changes have taken place during the process of manu- 
facture; and their competitors, products to ascertain what the 
latter are using. Analysts of this group may use methods which 
their knowledge of the subject tells them will give accurate 
results, even though it is known that such methods are not uni- 
versally applicable. The second group embraces the consumers, 
who, as a rule, are endeavoring to learn whether or not the article 
complies with certain stipulated requirements, or specifications, 



14 THE ANALYSIS OF RUBBER 

Here a definite procedure is obligatory, for in order to avoid dis- 
putes, the specifications usually (and if they do not, they 
should), 1 contain a more or less detailed description of the analyt- 
ical methods. Since the composition is unknown, it is clear that 
the methods in use by the consumers should be as nearly uni- 
versally applicable as it is possible to make them. From time to 
time, there have appeared suggestions for making analyses of 
rubber compounds, but too frequently the authors have neglected 
to take into consideration these different points of view, and this 
omission has materially reduced the value of the suggestions. 

Rubber is a hydrocarbon of the terpene family, existing in a 
polymerized form, and having the composition (C 10 H 16 ) X . The 
size of the molecule is unknown, although it is believed to be quite 
large, but we do know that each group of C 10 H 16 contains two 
double bonds. Double bonds are unstable, and there is always a 
tendency for such double bonds to add various elements, or 
group of elements, which will tend to produce a more stable 
form. Thus we find that the double bonds of rubber may take 
up oxygen, ozone, sulfur, selenium, sulfur chloride, chlorine, 
bromine, etc., producing new chemical substances with distinctly 
new properties, many of which are more useful in a commercial 
sense than the original substance. Industrially, the most im- 
portant of these compounds are those formed by the addition of 
sulfur, and sulfur monochloride, the chemical process being 
termed "vulcanization," or "curing." Crude rubber is a soft, 
plastic substance, soluble in naphtha, benzene, chloroform, carbon 
bisulfide, from which, by the simple process of evaporation, it 
may be recovered in its original form. The addition of compara- 
tively small amounts of sulfur is sufficient to destroy the solu- 
bility in these solvents. Such vulcanized compounds can, by pro- 
longed heating, be brought into solution in various solvents, but 
there is this distinction, that, in the latter case, the solution is 
accompanied by a depolymerization, and evaporation of the sol- 

1 Ttrere is such a diversity of opinion concerning the best method for any 
single determination, and since the interpretation of the analysis, rather than 
the absolute results obtained, is the more important of the two, it must neces- 
sarily follow that the results of the analysis are inseparable from the method 
by which they were obtained. It is not sufficient merely to say that a sample 
has 3.00% of sulfur, it must be stated that it has 3.00% of sulfur when deter- 
mined by a certain method. This has been one of the glaring weaknesses of 
the average specification in this country, and has been the cause of a great 
deal of controversy and actual financial loss. 



THE PURPOSE OF RUBBER ANALYSIS 15 

vent will not give us the rubber in the same condition in which it 
existed before solution. Rubber containing only a small quantity 
of combined sulfur is tough and elastic, but as the percentage of 
combined sulfur increases, the degree of extensibility becomes less 
and less, the rubber becomes harder until we obtain the familiar 
substance, vulcanite, or hard rubber, and the limit of sulfur addi- 
tion is found at (C 10 H 16 S 2 ) x . To effect the combination between 
the rubber and sulfur, catalysts are employed, both organic and 
inorganic, while to produce the desired properties in the finished 
article, various oils, waxes, gums and pigments are added. 

It would seem, therefore, to be quite apparent that in order to 
understand the analysis of rubber, one must be familiar with the 
materials which enter into the rubber compounds, the chemical 
changes which take place during vulcanization, as well as merely 
the analytical methods. In this way, the analysis may be 
directed towards bringing out the really important points in the 
rubber compound. The general scheme which has been adopted, 
is to first present a description of the raw materials, the methods, 
and the theories of vulcanization. This will be followed by direc- 
tions for sampling, general and specific methods of analysis, and 
finally some suggestions for interpreting the results of the analy- 
sis, with the view to reconstructing the formula of the rubber 
compound. 



Chapter II. 
The Composition of Crude Rubber. 

Crude rubber is obtained from various trees, shrubs, or vines. 
Some of these grow wild, and others are cultivated for the sake of 
their yield of rubber. Twenty years ago cultivated or plantation 
rubber was practically unknown; the crude rubber of that time 
was obtained from all quarters of the tropical world, Brazil fur- 
nishing the greater portion of the wild rubber. Not only did 
Brazil furnish the major part of the rubber, but it was also the 
best in quality, largely because of the care taken in preparation, 
and the uniformity achieved in spite of the rather crude methods 
which were employed. One reason for this uniformity was that 
most of the rubber was obtained from a single species, the Hevea 
Braziliensis, which today is not only the source of the best wild 
rubber, but, through transplanting, is also the chief, one might 
almost say the entire, source of the plantation rubber as well. 
The Hevea rubber became known commercially as Para rubber, 
from the port from which shipments were made. 

Para Rubber. Two main subdivisions are made in Para rub- 
ber, the Up-river, and Islands. The former comprises rubber 
collected in the inland section, along the Amazon river and its 
branches. The Islands rubber is so named because it is largely 
collected in the islands of the delta of the Amazon, and the ad- 
jacent country. There are subdivisions of the main group, the 
Up-river including Acre, Bolivian, Madeira, Manaos, etc., and 
the Islands rubber is similarly subdivided. 

Rubber comes from the latex of the trees, and the latex is 
gathered by making a number of small cuts extending just below 
the bark. The latex flows from these cuts, and is caught in small 
cups. The rubber gatherer collects the latex daily, takes it to his 
hut, and prepares it for the market by the process of coagulation 
and smoking. A paddle is dipped into the latex, and the thin 
film which remains on the surface is coagulated by holding it over 

16 



THE COMPOSITION OF CRUDE RUBBER 17 

the smoke from burning uri-curi nuts. The heat and smoke break 
down the emulsion, separating the rubber from the so-called 
serum of the latex. Much of the serum drips out, but a consider- 
able portion is retained, and the solids contained therein become 
a part of the crude rubber, profoundly influencing the vulcaniza- 
tion and the physical properties. The operation of dipping (or 
the latex may be poured over the paddle) and smoking is con- 
tinued until a fair sized ball is obtained. The rubber so prepared 
is called Fine Para, but if for any reason fermentation or oxida- 
tion should set in, and the rubber become sticky, it is classed with 
the lower grades. The scrap from the cups, buckets, and from the 
bark of the trees, is gathered together, and called "Coarse 
Para." The shape and general appearance of these "balls" varies 
widely, but the method of preparation is the same throughout, so 
that there actually exists a remarkably uniform method of prep- 
aration throughout the entire territory where the Para rubber is 
gathered. 

Castilloa. Second among the wild rubbers is that obtained 
from the Castilloa Ulei or Castilloa Elastica which produce the 
kinds known as Caucho, Centrals, etc. This rubber is coagulated 
in bulk, is not smoked, and appears on the market as balls, 
sheets, strips, or slabs. It is subdivided into grades, but, even in 
the best, there is nothing like the uniformity of quality which 
one finds in the Para rubber. 

African Rubbers. African rubbers are largely gathered from 
vines, chiefly the Landolphia, with innumerable sorts and grades 
many of which are quite indistinguishable, even to the expert, and 
certainly cannot be identified after vulcanization. Some African 
rubbers are prepared according to methods peculiar to the place, 
by means of which they can be identified, or they may gather 
from the means by which they are coagulated an odor peculiarly 
their own, but the differences from one lot of the same name to 
the next is frequently greater than that between two entirely 
different sorts. During the past year there has been a decided 
diminution in the quantity of African rubber produced, and many 
sorts have entirely disappeared from the market as their quality 
is so poor that the price they will bring on the present day 
markets is not sufficient to pay the cost of collecting. If the 
Plantation rubber continues to increase at anywhere near the 
rate it has for the past eight or ten years, it will mean such a low 



18 THE ANALYSIS OF RUBBER 

standard market price for the best grades of rubber that the 
poorer African sorts will disappear altogether. From the experi- 
ences which the manufacturers have had in trying to produce 
uniform quality material with such stuff, we may surmise that 
no tears will be shed at the loss. 

Guayule. Guayule is the rubber obtained from the shrub 
Parthenium Argentatum, found extensively in Mexico and Texas. 
This rubber is not obtained in the form of a latex, but the 
plants are cut down, and the rubber which exists in the stems, 
leaves, and branches of the plant, is separated by mechanical or 
chemical means, or both. The crude Guayule thus obtained runs 
very high in resins and other impurities ; indeed, these form about 
two thirds of the crude rubber. It usually undergoes a process 
of purification, or deresinification in order to prepare it for the 
market, whereby the rubber hydrocarbon content is raised to 
somewhere around 75%, or even higher. Guayule is a soft, 
sticky, stretchy rubber, retaining these properties to a high 
degree even after vulcanization, and it finds its chief use as a con- 
stituent of frictions. 

Pontianak. Java, Borneo, and the neighboring countries, pro- 
duce a tree, the Dyera Costulata, which yields a product contain- 
ing about 90% of resins and similar substances, and about 10% 
of rubber. This mixture is known chiefly as Pontianak, or Jelu- 
tong rubber. In the crude form, it is quite hard, owing to the 
high resin content, and particularly to the nature of the resin. 
In the process of purification of crude Pontianak, a large part of 
this resin is removed, and is marketed separately. Pontianak 
resin finds some use in rubber mixings; it is hard, brittle resin, 
with a conchoidal fracture, very much resembling our ordinary 
rosin. It is soluble in acetone, chloroform, benzene, and other 
organic solvents, and consists largely of unsaponifiable hydrocar- 
bons. Ellis and Wells 1 find that on heating, the solubility of the 
resin and the percentage of unsaturated compounds increase. 
While there is some demand commercially for this resin, it does 
not appear to be sufficiently extensive and remunerative to permit 
much Pontianak rubber to come to this country. At the present 
prevailing market prices, it seems obvious that the rubber portion 
must be handled as a by-product only. 2 

1 J. Ind. Eng. Chem. 7, 747-50 (1915). 

2 As an indication of the disappearance of Pontianak rubber from the market, 
it is only necessary to note that according to reasonably reliable statistics, only 



THE COMPOSITION OF CRUDE RUBBER 19 

When the resin content is materially reduced, Pontianak rub- 
ber is very tacky, and plastic, making it difficult to store, as it 
has the tendency to flow together to form one huge, unmanageable 
mass. 

Plantation Rubbers. The development of the Hevea on the 
plantations of the Far East, has reached such proportions as to 
make it the dominating feature of the rubber market. Fifteen 
years ago, plantation rubber was of small commercial importance, 
very little of it being produced. Today, the plantations furnish 
fully 80% of the world's supply. The rapidity of the growth is 
well illustrated in the following figures, which while they may not 
be absolutely accurate, are sufficiently so to show the rapidity of 
the growth of this phase of the industry : 

Production of Plantation Rubber. 

Tons 

1903 25 

1904 50 

1905 150 

1906 500 

1907 1,000 

1908 2,000 

1909 4,000 

1910 8,000 

1911 15,000 

1912 30,000 

1913 50,000 

1914 75,000 

1915 110,000 

1916 160,000 

1917 225,000 

1918 190,000 

1919 360,000 

The time has arrived when cultivated rubber can be produced 
so cheaply that the poorer grades of wild rubber have been 
forced out of the market, and even the better grades have suffered 
severely. The analyst may therefore expect less and less to be 
confronted with samples for analysis which have been made up 
wholly, or in great part, of wild rubbers. Only in the Para grades 
does there seem to be any sort of adherence to the old grades of 
wild rubber. There are still some specifications for various ma- 
terials, which insist upon the use of Fine Para rubber (although 
unless some representative of the purchaser actually sees the 

1000 tons were imported during 1921. During the same period, crude rubber 
imports were estimated to be between 275,000 and 300,000 tons. In 1905-6, the 
ratio of imports was 2 tons of crude rubber to one ton of Pontianak. 



20 THE ANALYSIS OF RUBBER 

material made, how they are going to distinguish good smoked 
sheets from Fine Para is more than one can say), and they are 
unwilling to change over to plantations because they do not know 
what the effect of such a change would make on the life of the 
articles. Some rubber specialties have been made from the same 
formulas, calling for Para grades, for a number of years, and still 
continue to be made in this fashion, although at times it is 
difficult to get just the grades of wild rubber needed. 

Smoked Sheet. Although at times it does not command the 
highest price, it is the standard grade of plantation rubber. 3 The 
rubber should be clean, dry, firm, of a good color and free from 
more than traces of mold or rust. The moisture content will vary 
between 0.3% and 1.0%. The acetone extract will usually be 
between 2.5% and 3.0%, and almost always will be below 4%. 
The ash should be negligible. 

Pale Crepe. Pale crepe is frequently called first latex, al- 
though the same latex may, at the choice of the plantations, be 
made into either ribbed smoked sheet, or pale crepe. The latter 
is usually cleaner than smoked sheet; chemically, they are very 
much alike. The moisture content will average lower than 
smoked sheet, the ash is negligible, and the resin content between 
2.5% and 4.0%. 

Smoked Crepe. Smoked crepe is usually cleaner than smoked 
sheet (the latter frequently contains bark, etc.), with a lower 
moisture content, approaching that of pale crepe. The resins 
seem to run about the same; if anything, a bit higher than the 
average of smoked sheets. No other particular differences have 
been noted. 

Amber Crepe. Amber crepe comes in several grades, according 
to color. There is no sharp dividing line between these grades 
and the pale crepe, or even amongst themselves. Some of the 
lighter amber crepes are very much like the poorer lots of pale 
crepe. The resins, moisture, and ash in the paler colored amber 
crepes is about the same as for pale crepe or smoked sheet; the 
lower grades are apt to be sticky, run high in dirt and moisture, 
and by reason of surface oxidation, they may be tacky and show 
a higher acetone soluble figure. 

Roll Brown Crepe. Roll brown crepe comes into the market 

3 For the methods of preparation of Smoked Sheet, and Crepe, cf. Whitby, 
"Plantation Rubber, and the Testing of Rubber." 



THE COMPOSITION OF CRUDE RUBBER 21 

in the form of sheets of crepe which have been rolled up into 
small bundles about 5 to 6 inches in diameter, and about 10 to 15 
inches in length. It is the lowest grade of plantation rubber on 
the market, is very tacky, and dirty, and must always be washed 
in the factory before it can be used. When washed clean, and 
dried, it replaces acceptably the wild rubbers which have been 
used in friction stocks, such as Guayule, etc. 

Constituents of Crude Rubber, Other Than the 
Rubber Hydrocarbons. 

We have already drawn attention to that portion of the crude 
rubber which is soluble in acetone, and which is known com- 
mercially as rubber resins. Apart from the dirt, bark, and water, 
which may be included in crude rubber, but which we cannot 
consider as anything but contamination, there are some other sub- 
stances, which are not rubber, but are nevertheless found in all 
crude rubbers. 

Resins. Hevea rubber contains, in addition to the rubber 
hydrocarbons from 2% to 4% of resins. These resins are about 
80% saponifiable, and 20% unsaponifiable. They are soluble in 
acetone, alcohol, chloroform, and many other organic solvents. 
The solution is usually a pale yellow color, and the residue, when 
the solvent has been driven off, is light colored with the consist- 
ency of butter. In the unsaponifiable portion, Whitby* has 
identified some five substances from the unsaponifiable portion, 
some of which show optical activity, and some give sterol reac- 
tions. The acetone extract of Hevea rubber may go higher than 
4%, but this does not necessarily mean that the resin content is 
high, but rather that there has been oxidation and depolymeriza- 
tion of the rubber, producing by-products which also are soluble 
in acetone. 

Insoluble Matter. If we take a sheet of pale crepe, smoked 
sheet, etc., and dissolve it in gasoline, being careful not to shake 
too much, we will find flakes of the crude rubber which will not 
dissolve. This is what is known as the "insoluble matter." The 
amount will vary with the method of preparation; analyses have 
run between 2% and 6%. Rubber prepared by the total evapora- 

* Paper read at the Spring meeting of the American Chemical Society at 
Rochester, April 1921. "Contribution to the knowledge of the resins of Hevea 
rubber," by G. Stafford Whitby and J. Doolid. 



22 THE ANALYSIS OF RUBBER 

tion of the latex will have the highest figure, whereas the ordinary- 
methods of coagulation with acetic acid, washing, etc., reduce this 
figure considerably. The insoluble matter resembles the proteins, 
and, according to Eaton, its fermentation will permit the forma- 
tion of nitrogenous decomposition products which act as acceler- 
ators of vulcanization. Such reactions take place in the so-called 
slab rubber, in which the coagulum is only slightly pressed, and 
which retains a large amount of the non-soluble substances in 
the latex. 

While the insoluble matter may be shown by treating the orig- 
inal sheet with gasoline as described above, it is next to impossible 
to wash out all of the rubber, so that we cannot depend upon 
this separation as a means of a quantitative separation. The 
nitrogen factor is obtained by dividing the weight of the nitrogen- 
containing substance by the nitrogen it contains; one determines 
the nitrogen and multiplies by this factor to arrive at the total 
amount of nitrogen-substance present. This factor varies con- 
siderably, but 6.25 is a fair average, and will give results near 
enough to the truth to be acceptable for all practical purposes. 
In the determination of glue by the Kjeldahl method, this in- 
soluble matter appears as a conflicting element in the determina- 
tion, and must be taken into account. 

The best rubbers are clean and dry, and have practically no 
ash. A high ash indicates a rubber which has been poorly 
washed or which has since picked up dirt, sand, etc. 

There are usually small amounts of substances, whose composi- 
tion we do not know, but which we recognize by the fact that they 
act as accelerator of the vulcanization process. In amount, they 
are negligible, except in the case of compounds composed entirely 
of rubber and sulfur, when their presence or absence may bear an 
important part in securing the proper degree of vulcanization. 

Tests for Crude Rubber. 

Crude rubber may contain dirt, bark, moisture, resins, proteins, 
and oxidized or depolymerized rubber. Bark, dirt, moisture, and 
any water-soluble substances, are grouped together as "loss on 
washing." 

Loss on Washing. For plantation rubbers, in which the mois- 
ture and dirt is usually very low, a 5 lb. to 10 lb. sample will suf- 



THE COMPOSITION OF CRUDE RUBBER 23 

fice. The sample should be taken in small pieces from different 
parts of the lot, and at least every five cases should be sampled. 
If the sample thus taken proves to be too large to handle, it can 
be weighed, broken down on the mill, and a smaller sample taken 
from this broken down rubber. The latter should be weighed 
when cool, in order to ascertain whether or not any loss in weight 
has taken place. For wild rubbers, not less than 50 lbs., and pref- 
erably 100 lbs. should be taken for the loss in washing test; 
afterwards, for the other determinations, a smaller sample may 
be drawn from the washed and dried rubber. Even greater care 
must be exercised in sampling wild rubber, because of the uneven- 
ness in size, cleanliness, moisture, etc., of the various balls or lots 
of wild rubber. Fine Para, for example, may be sampled by cut- 
ting the balls into quarters, until about 50 lbs. are obtained. 
Dirtier rubbers, or those which will vary more from lot to lot, 
should be sampled up to 100 lbs. In a later chapter, we propose 
to deal more at length with this subject of sampling, but suffice 
it to say here that unless the proper care is exerted to make the 
sample drawn for this test one which is of the same average 
quality as the lot, the entire work of testing is worse than if it 
were not done at all, for it may lead to totally false results. The 
rubber should be washed immediately after the sample has been 
drawn and weighed. 

Plantation rubber may be washed directly, without any pre- 
vious treatment; wild rubbers should be heated in hot water to 
soften them, and render them more plastic, so as to facilitate the 
operation. The rubber is washed in the usual factory manner, 
and then dried in a vacuum dryer. After removal from the 
vacuum dryer, the rubber is cooled, and weighed, and the loss 
noted. 

A new sample of about 1000 grams is taken from different parts 
of the washed and dried sample, and united by passing several 
times through a laboratory mill. Five grams are weighed out, 
sheeted thin on the laboratory mill (care must be taken to see 
that no mechanical loss occurs), and dried to constant weight at 
100C. A laboratory vacuum oven may be used, but the tempera- 
ture should be less than 100C, since with the reduced pressure 
the higher temperature is not necessary, and there is less likeli- 
hood of damage to the rubber at the lower temperature. The loss 
on drying the 5 gr. sample, plus the shrinkage during washing, 



24 THE ANALYSIS OF RUBBER 

gives the total loss in weight, and should be calculated to per- 
centage, based upon the original weight of the sample. 

Resins. Sheet out thin, 5 gr. of rubber, 5 calculated to the 
dry basis, and wrap in filter paper which has previously been 
extracted with acetone, place in the extraction flask, 6 and extract 
continuously with acetone for eight hours. Remove the solvent, 
dry the flask and contents to constant weight at 90C and calcu- 
late to percentage. The color, hardness, and odor of the extract 
should be noted. 

Moisture. It is sometimes desirable to know simply the mois- 
ture in the original sample. This is not practicable with most 
wild rubbers, where the moisture is very unevenly distributed, 
but with plantation rubbers it is quite feasible, and often a 
valuable figure. 

Cut up 5 grams into small pieces, dry to constant weight in an 
inert atmosphere at 90C. Calculate to percentage. 

Nitrogen. A 1 gr. sample is placed in a Kjeldahl flask, with 
10 gr. of potassium sulfate, 50 cc. of cone, sulfuric acid and 1 gr. 
of copper sulfate. Heat for three to four hours (it is not neces- 
sary for the solution to become clear), transfer to a distilling 
flask, make the solution alkaline with caustic soda, and distil the 
ammonia into standard sulfuric or hydrochloric acid. Titrate the 
excess of acid with standard sodium carbonate, using methyl 
orange or methyl red as indicator. 

Various determinations on the amount of nitrogen in the in- 
soluble matter, have given figures ranging between 12% to 16%. 7 
The usual factor of 6.25 will give a conservative figure for the 
proteins, but it is likely that 8.0 or even higher, may frequently 
be the more correct value. It will be seen from these figures that 
the determination of nitrogen does not signify very much. 

Curing Tests. It is desirable not merely to know the chemical 
composition and the loss on washing of crude rubber, but also to 
know something of its vulcanizing properties. For this purpose, 
a standard formula should be employed, a series of cures made 
from this mix, and stress-strain curves drawn for each cure. 

B It is convenient, if not pressed for time, to take the dried rubber from the 
moisture determinatibn in loss on washing. This simplifies the correction, but 
in so doing, it must be seen that the sample has not been altered during the 
drying, by oxidation, or depolymerization. 

6 Cf . Acetone extraction, under methods of analysis, page 68. 

» Cf. Schmitz, Gummi Ztg. 27, 1085. 1131 ; Spence and Kratz, Koll. Zeit. 1J,, 
262-77 (1914). 



THE COMPOSITION OF CRUDE RUBBER 25 

The question of a standard formula is one which may nut be 
dismissed lightly. At present, many of the plantation and factory 
chemists are using a mixture of rubber and sulfur. This, how- 
ever, is open to serious objection, 8 and a less objectionable pro- 
cedure, even granting that the formula itself may not be the 
best one, or most suited for all work, is to use a formula contain- 
ing a small amount of zinc oxide, and sufficient accelerator and 
sulfur to produce satisfactory cures. One such formula would 
be: hexamethylenetetramine 0.5%, sulfur 4.5%, zinc oxide 5%, 
rubber 90%. This mixture contains enough sulfur for a 
coefficient of 5.0, 9 which is higher than one would ordinarily go, 
and zinc oxide in excess of that required to neutralize any organic 
acids in the rubber, and provide a basic mix for vulcanization, 
since practically all organic accelerators seem to work better 
under such conditions. 10 Particular pains should be taken regard- 
ing the quality of the zinc oxide, sulfur, and accelerator; they 
should be of C. P. grade, and not just the commercial stuff used 
in the factory. Such grades are to be found in the market, and 
are worth the extra cost. It is not without the bounds of reason 
that much of our unexplainable vagaries in rubber testing is 
really traceable to the impurities in the pigments, and not to the 
rubber itself. 

Needless to say, perhaps, the results depend largely on the 
cleanliness and technique in mixing and curing, the accuracy of 
the thermometers, and the accuracy of the testing machines. No 
tests should be made until at least 48 hours after vulcanization. 11 

8 Cf. J. B. Tuttle, Variability of Crude Rubber, J. Ind. Eng. Chem. is, 519-22 
(1921). 

•The sulfur coefficient, sometimes called the coefficient of vulcanization, is 
the ratio of combined sulfur to the rubber. It is calculated by dividing the 
percentage of rubber by the percentage of combined sulfur. 

It may be mentioned that the coefficient of vulcanization necessary to pro- 
duce identical physical properties in two or more compounds, is not a constant, 
but varies with the amount and nature of the accelerator employed, and to a 
lesser extent on the other constituents of the compound. 

10 The real purpose of the use of the added organic accelerator and the zinc 
oxide should not be lost sight of in any discussion of the advisability of using 
this or any similar formula. It has been shown that crude rubber contains 
varying amounts of natural organic accelerators, and we must eliminate their 
effect if we are to study the actual variation of the rubber itself. 

u There are some who believe that 24 hours is sufficient to permit the rubber 
samples to reach equilibrium. At times, we have taken samples from the vul- 
canizing press, and after cooling in running water, tested them immediately. 
But where results of today are to be compared with those of the past, or with 
those to be obtained in the future, the only safe procedure is to allow the full 
48 hours, so that such comparisons as may be made will be made under identical 
circumstances, and any differences noted will be real ones, and not those caused 
by the fact that at times samples had not yet reached equilibrium. 



Chapter III. 
Tlie Preparation of Rubber Compounds. 

The art and science of preparing rubber compounds is some- 
thing which may well deserve treatment of its own. It is not 
the intention to explore the whys and wherefores of the matter, 
for many of the commercial compounds just "grew up" as time 
went on, a little of a new material here, and a little less of an old 
one there, until at present they are so complicated that even the 
owners of the formulas are afraid to make any further alterations. 
On the other hand, we have a very large number of formulas 
which have been constructed on the basis of the definite physical 
and chemical properties of such a mixture as determined by 
years of research. Irrespective of why it was used, the analyst is 
primarily interested only in what materials are likely to be 
used. 1 Moreover, it is utterly impossible to include every article 
which has ever been used in rubber manufacture, but only those 
which have really attained some commercial importance, and 
hence are likely to be encountered in an analysis. 

Crude Rubbers. In the preceding chapters, the general proper- 
ties of the most important crude rubbers were given. This is 
probably as good a time as any to draw attention to the fact that 
seldom will one find a single kind of crude rubber in a rubber 
compound. Coarse Para will be mixed with Fine Para, or amber 
crepes will be mixed with smoked sheets or pale crepe. It may 

'At the time of writing, the situation with respect to crude rubber is such 
that the preparation of a new compound is a more than usually serious prob- 
lem. With the best grades of plantation rubber selling around 15 cents a pound, 
the saving in the use of reclaimed rubbers and substitutes is questionable, if 
we consider that such materials are to replace the rubber. Some reclaimed 
rubbers may have an added value on account of the active fillers, such as zinc 
oxide and gas black or lamp black, which they may contain ; or we may use 
reclaims and substitutes in special cases on account of special properties which 
they impart. However, it is incredible that such conditions as now prevail are 
to continue indefinitely, and hence we are proceeding on tbe basis that the 
normal price for crude rubber will be from 25 to 30 cents (if not higher), and 
at this price the use of certain grades of reclaims and substitutes will effect 
savings in costs, and bence the analyst may expect to find them in examining 
manufactured articles. 

26 



THE PREPARATION OF RUBBER COMPOUNDS 27 

seem superfluous, but it is safer to call attention to the fact that 
replacing Fine Para with Coarse Para, or smoked sheet with am- 
ber crepe, is merely a matter of economy ; the rubbers used are not 
as good as those they replace, and the quality of the compound is 
lowered. It is purely a question of deciding whether or not the 
properties of the compound are sufficient to meet the demands 
of the service. On the other hand, rubber such as Pontianak, 
Guayule, roll brown crepe, etc., when used as softeners, are used 
independently of their cost, and their use has continued in many 
cases when they cost practically as much, or even more than the 
so-called better rubbers. These points are worth bearing in mind 
in figuring out the probable formula from the analysis of a 
rubber compound. 

Reclaimed Rubber. We have seen that the rubber hydrocarbon 
can combine with sulfur until the compound (C 10 H 16 S 2 ) x is 
reached, when the ratio of rubber to sulfur is 136:64. In the 
ordinary soft vulcanized articles, the sulfur coefficient is between 
1.5 and 5.0, depending upon the type of accelerator, and the 
degree of vulcanization. Such material is able to take up fur- 
ther quantities of sulfur to form a new compound with a higher 
coefficient, which, while somewhat harder than the material from 
which it was made, may still be of service. Each addition of 
sulfur, other conditions being equal, produces a harder product 
than before, until, with the maximum amount of sulfur which 
may be added, we reach the product ebonite. The hardness of 
the rubber itself is frequently lessened by the admixture of soft- 
ening oils, and the partial depolymerization which takes place 
produces a soft and tacky substance, which also helps to counter- 
act the hardening effect of the additional sulfur. 

Before vulcanized rubber can be used a second time, it must 
be put into condition to be mixed in a homogeneous manner with 
new rubber. There are two general processes employed, (a) the 
acid reclaiming process; and (b) the alkali reclaiming process. 
These processes serve to remove any fabric which may be present, 
the free sulfur, and, of course, some of the fillers, both organic 
and inorganic. In the latter case, the amount and nature of the 
fillers removed will depend largely upon the process which is used 
and the chemical nature of the fillers. Zinc oxide and whiting 
are largely removed in the acid process, zinc oxide to a slight 
extent in the alkali process, while gas black and lamp black are 



28 THE ANALYSIS OF RUBBER 

unaffected by either. Oil substitutes are not attacked in the 
acid process, but are almost completely removed by the alkali 
process. 

These processes of reclaiming do not reverse the vulcanization 
process; on the contrary, if there be any quantity of free sulfur 
present, part of it will combine with the rubber during the re- 
claiming, the sulfur coefficient being higher afterward than before. 
Other processes have been worked out for the purpose of taking 
out the sulfur and restoring the double bond, in which case we 
would expect a product similar to new rubber, and which would 
vulcanize in the same manner. This is the ideal towards which 
the researches have been directed, but it must be admitted that 
as yet we have fallen far short of the ideal, and the reclaimed 
rubber encountered in vulcanized compounds has been made by 
one or the other of the two methods mentioned above, or some 
variation of them. 

Reclaimed rubber is added, under normal market conditions, 
first of all because it is cheaper. Certain grades may be used 
because they give desirable properties; for example, vulcanized 
reclaimed rubber resists oil better than does new rubber, and the 
use to which the article is to be put is worthy of notice in deciding 
whether or not reclaimed rubber has been used on account of its 
cost, or because in the case in question, it is actually better. 

In the manufacture of pneumatic tires, there is always a con- 
siderable amount of fabric trimmings, containing a large amount 
of new, unvulcanized rubber. By the acid reclaiming process, the 
fabric may be entirely removed, with a considerable portion of 
the sulfur, without appreciably causing the rubber and sulfur to 
combine. The product, known as "reclaimed or pure gum fric- 
tion," is a valuable adjunct in rubber compounding. 

Oil Substitutes. 

In the preparation of certain articles, where the highest physi- 
cal properties were not of primary importance, substitutes for 
rubber have been used in order to lessen the cost of manufacture 
(cf. footnote, page 26). One group of such substitutes is made 
from oils of various kinds, and these substitutes are known com- 
mercially as "oil substitutes." 

When drying, or semi-drying oils, such as linseed, soya, corn, 



THE PREPARATION OF RUBBER COMPOUNDS 29 

cottonseed, and similar oils, are treated with sulfur or sulfur 
chloride, a solid plastic mass is obtained. These products have 
been called vulcanized oils, because of the similarity of the 
processes of preparation with those of rubber. The reaction with 
sulfur requires heating, and the product varies in color from a 
light to a very dark brown, or even black. The sulfur chloride 
combines at ordinary temperatures, giving us the so-called "white 
substitutes." 

Mixed with these substitutes are various gums and oils, pro- 
ducing an almost endless number of combinations. This need not 
bother the analyst, however, for the treated oils are insoluble in 
acetone and chloroform, whereas the untreated oils and gums are 
usually soluble in one or the other of these solvents. They may 
also be loaded with mineral pigments of various kinds. 

Tests of Oil Substitutes. 

An examination of the raw material should cover the un- 
changed oil, loss on heating at 100C, free sulfur, and ash. Un- 
changed oil acts in a totally different manner from the true 
substitute, and the free sulfur is especially important, since it is 
capable of combining with the rubber during vulcanization; 
hence any free sulfur present must be taken into account when 
figuring the amount of sulfur to be added as such to the rubber 
compound. 

Unchanged Oil. Reduce the sample to a fine state of division 
by crumbling or cutting. Extract 2 gr. with acetone for eight 
hours; dry the extract to constant weight at 90C, cool and weigh. 

Free Sulfur. Treat the dried acetone extract with 50 to 75 cc. 
of water, and 2 to 3 cc. of bromine, heat until colorless, or nearly 
so, filter through a folded filter; heat the filtrate to boiling, add 
10 cc. 10% barium chloride, and determine the precipitated 
barium sulfate as usual. Calculate to sulfur, and deduct the 
percentage of free sulfur from the total acetone extract. The 
remainder is the unchanged oil. 

Loss in Weight. Dry a 2 gr. sample in a neutral atmosphere at 
90/100C until constant weight is secured. 

Mineral Fillers. Ignite a 1 gr. sample, cool the residue and 
weigh. Pure oil substitutes should have practically no ash; if 
any pigments are added, the amount will be such as to leave no 



30 THE ANALYSIS OF RUBBER 

doubt in the analyst's mind as to whether such additional was 
accidental, or not. Oil substitutes are usually found in amounts 
of from 1% to 5%, although we have seen some German made 
rubber tubing that had nearly 50% of oil substitute. 

Mineral Rubber. 

The mineral rubbers are asphaltic or bituminous hydrocarbons 
obtained either from natural or artificial sources. The natural 
sources are from the minerals gilsonite and elaterite, while the 
artificial mineral rubbers are made largely from the blown oils 
from petroleum residues. 2 

Mineral rubber possesses a melting point above that of the 
usual vulcanization range, but its plasticity enables it to be 
worked readily at much lower temperatures. In amounts up to 
7 volumes, 3 it materially improves the tensile properties. It 
serves to soften the uncured stock, makes it tackier reduces 
blooming, and in a variety of ways proves itself to be an asset to 
a rubber compound. It improves the waterproofing properties 
of rubber. 

Owing largely to the differences in the source of supply, and to 
the various methods of preparation, the chemical and physical 
properties vary widely. The acetone-soluble matter varies enor- 
mously, running as low as 17%, and as high as 60%, the higher 
percentages being the more common occurrence. Chloroform will 
dissolve part of the residue, equal to about 10% of the whole. 
They may contain as much as 10% of their weight in sulfur, all of 
which is chemically combined. There is always a fair sized 
amount which is soluble neither in acetone nor chloroform. 

While the solvents do not give us exact data as to the quantita- 
tive figures on mineral rubber, the color of the chloroform extract 
is a very reliable index in determining the presence or absence of 
this material. When present, this extract is deep brown to black 

3 For the best and most recent work on Mineral Rubber, consult the article 
by C. Olin North, "Mineral Rubber," read at the meeting of the Rubber Division 
of the American Chemical Society at New York, September 6th to 10th, 1921. 
Abstracts of this paper are to be found in the "India Rubber World." 65, 191-2 
(1921), and "The Rubber Age," 10, 130-1 (1921). 

s Since the specific gravity of the materials used in rubber compounding varies 
widely, it affords a more exact method of comparing the effect of the different 
substances if they are compared on the basis of volume rather than weight. 
The volume is referred to the total volume of rubber, the latter being taken 
as 100, 



THE PREPARATION OF RUBBER COMPOUNDS 31 

in color, and is not likely to be confused with any other class of 
material used in rubber manufacture. 

During vulcanization, the percentage of soluble matter may 
change somewhat; the acetone extract is usually somewhat lower 
than when the material itself is subjected to extraction. The 
chloroform extract shows little change. Various explanations 
have been offered: (1) that the mineral rubber unites with the 
rubber; (2) it combines with the sulfur to form insoluble prod- 
ucts; (3) the dispersion of the mineral rubber on the crude rubber 
produces an adsorption effect, and renders the former more diffi- 
sult to dissolve out of the mix. Of these, the second seems to be 
the most plausible, although admittedly the other two are 
possibilities. 

Mineral rubber has a specific gravity of about 1.00; the hard- 
ness varies according to the melting point. The melting point 
is anything that may be desired, but the most popular grade is 
that melting in the neighborhood of 310F. 

North 4 has determined that the best results with mineral rub- 
ber are obtained when the proportion is 7 volumes of mineral 
rubber to 100 of rubber. One is more likely to meet with less 
rather than with more than this amount. 

Tests for Mineral Rubber. 

Acetone Soluble. Extract with acetone for four hours, a 1 gr. 
sample of the mineral rubber; dry to constant weight, at 100C. 

Chloroform Extract. Without drying the sample which has 
been extracted with acetone, extract with chloroform for two 
hours, or longer if at the end of that period the solvent is still 
colored. Dry the extract to constant weight, at 100C. 

Ash. Ignite 1 gr. in a porcelain crucible, cool and weigh. The 
residue should be negligible. 

Insoluble Matter. The difference between 100% and the sum 
of the acetone and chloroform extracts, and the ash, shall be 
called "insoluble matter." 

Mineral Hydrocarbons. 

The mineral hydrocarbons may be divided into two classes, 
hard and soft. The former include ozokerite, ceresin, and par- 
affin; the latter, petrolatum and heavy mineral oil. The hard 

*Loc. cit. 



32 • THE ANALYSIS OF RUBBER 

hydrocarbons are useful for their waterproofing effect, and are 
to be found largely in materials intended for electrical purposes, 
such as insulated wire and cable. The soft hydrocarbons are used 
purely as softeners, to facilitate the handling of the stocks in the 
factory, and whereas the hard hydrocarbons are without any 
serious effect on the aging qualities, the soft hydrocarbons have a 
decided deteriorating effect, and must be used in small quantities. 
The explanation of this effect would appear to be that the mineral 
oils are solvents for vulcanized rubber (as previously stated, 
however, this is not a true solution, but rather a depolymerization 
preceding solution). 

Mineral hydrocarbons are rarely used to a greater extent 
than 5%, and in the greatest number of cases the amount used 
is between 1% and 2%. 

Ozokerite. Ozokerite is a natural product, found in Austria, 
Russia and southern Utah. It is dark brown to black in color, 
with a specific gravity of about 0.90. The melting point should 
exceed 65C (150F). Ceresin is ozokerite which has been purified 
by treatment with fuming sulfuric acid; it is pale yellow in color, 
with a resinous luster, non-crystalline in appearance, but in other 
respects, similar to the parent substance. 

Paraffin. Paraffin is a hard, white, crystalline substance, com- 
posed of the higher boiling hydrocarbons from petroleum. Its 
specific gravity is about 0.90, the melting point almost anything 
that one desires, from soft paraffin which borders closely on 
petrolatum, to the hard paraffins with melting points around 60C. 
Ozokerite and ceresin are so much higher in price than paraffin, 
that the temptation for adulteration is very great, and this is all 
the more true because of the fact that paraffin, which is used 
largely as the adulterant, is so near in chemical and physical 
properties that rather large amounts can be added without fear of 
detection. Ceresin in the pure state is much less crystalline than 
paraffin, and less brittle, but it is doubtful if these advantages 
warrant the extra cost of the pure article. 

Paraffin and ceresin have the peculiar property of working 
toward the surface of a rubber article, much in the same manner 
as sulfur "blooms." It appears within a few days after vulcani- 
zation, and if a slab of rubber containing paraffin be left un- 
touched for say six months, it is possible to scrape a considerable 
quantity of clean paraffin from the surface (possibly mixed with 



THE PREPARATION OF RUBBER COMPOUNDS 33 

sulfur if the free sulfur is high). This fact is important in 
analyzing such materials, for the ordinary handling, cleaning, 
etc., in preparing a sample for analysis, will remove an appre- 
ciable quantity, and hence, on this account, irrespective of the 
errors of analysis, the determination of paraffin or ceresin is 
likely to be low rather than high. 

Oils and Waxes. 

Rubber compounds may be made suitable for calendaring, tub- 
ing, and other operations, either by excessive working on the 
mixing mills, or by the use of elevated temperatures. Both 
methods are objectionable in one sense or another, the excessive 
working breaks down the rubber, producing a sticky, porous 
material which is difficult to handle, to say nothing of its poorer 
tensile properties. High temperatures are to be avoided in the 
preliminary stages of manufacture, especially with organic accel- 
erators, since some of the latter become very active at moderately 
low temperatures, and a partial vulcanization will be effected 
(what is technically known as "burnt" or "scorched" stock). 
One method for avoiding these difficulties is to add a small 
amount of oil (usually 1% to 3%), which softens the rubber 
compound and brings about satisfactory working conditions. 
We recognize two classes in these softeners, (a) in which the 
oil or wax acts merely as a softener; (b) in which in addition 
to its softening effect, it adds some distinct and desired property, 
such as tackiness, etc. In class (a) we find palm oil, cottonseed 
oil, petrolatum or vaseline, and heavy mineral oils; in class (b) , 
Burgundy pitch, colophony or ordinary rosin, rosin oil, tar oils, 
etc. The former may be expected in almost any stock, but the 
latter are used chiefly in cement stocks, frictions, tapes, etc., 
where adhesive properties have a particular value. 

Palm Oil. Palm oil is obtained from the fruit of the palm tree, 
Eloeis guineensis, and the west coast of Africa is practically the 
only important commercial source of this oil. Specific gravity, 
0.921-0.925; melting point 27-42C, solidification point 37-39C, 
depending upon the age and origin of the oil. Iodine number, 
53-57; the commercial oil contains water, sometimes as much as 
7% ; other impurities up to 3%. It may be adulterated with bark 
and dirt, and, before using, palm oil is melted, and the clean oil 



34 THE ANALYSIS OF RUBBER 

skimmed from the surface. Palm oil is rarely adulterated with 
other oils or fats, hence it is usually sufficient to determine water, 
total impurities, and the solidifying point. The color varies from 
orange yellow to a dark, dirty red. 

Cottonseed Oil. Cottonseed oil is obtained from the seeds of 
the cotton plant, Gossypium, of which the principal species are 
G. Herbaceum in the United States, and G. Barbadense in Egypt. 
Choice crude oil should be free from water and foots, possess a 
sweet flavor and odor (i.e., should not be rancid), specific gravity 
0.922-0.925; solidifying point 3-4C; iodine number 105-110. 



Tests for Cottonseed Oil. 

The best known test for cottonseed oil is Halphen's color test, 
made as follows: 1-3 cc. of the oil is dissolved in an equal 
volume of amyl alcohol, to this is added 1-3 cc. of carbon bisul- 
fide holding in solution 1% of sulfur. The test tube is immersed 
in boiling water, and the carbon bisulfide driven off. A deep red 
color appears in about 30 minutes. The test depends upon the 
presence of some chromogenetic substances which are destroyed 
by high heating, so that rubber compounds containing cottonseed 
oil may not show this test after vulcanization. 

Petrolatum. Petrolatum, or vaseline, may be either light or 
dark colored. Its specific gravity is between 0.85 and 0.90. At 
ordinary temperatures, it is a soft paste, but at 40 to 50C it 
melts to a clear fluorescent oil. It is not altered in composition 
during vulcanization, and, unlike paraffin, it remains distributed 
throughout the compound after vulcanization, and does not bloom 
to the surface. 

Heavy Mineral Oils. The heavy mineral oils are purely soften- 
ers, but are more likely to be found as component parts of 
reclaimed rubber and substitutes, than actually added to com- 
pounds as such. They act in practically the same manner as 
petrolatum. 

Burgundy Pitch. Burgundy pitch is more important for its 
adhesive properties than as a softener, although it acts in both 
capacities. It is a dark, brittle substance, with a resinous luster, 
and a specific gravity of about 1.10. It is soluble in acetone. 
It is obtained from the Norway spruce, Picea Excelsa, by scarifi- 
cation of the trees, and collecting the resin after it has hardened. 



THE PREPARATION OF RUBBER COMPOUNDS 35 

The volatile oils which are present in the crude resin are removed 
by boiling with water. It contains considerable bark and dirt, 
and must be purified by melting and filtering through sieves. 
It is frequently found in low grade frictions, insulating tape, 
cements, etc. 

Burgundy pitch is composed largely of abietic anhydride, and 
gives a positive reaction with the Liebermann-Storch test. It is 
so near ordinary rosin in composition that the latter is fre- 
quently used as an adulterant, and it is one that is exceedingly 
difficult to detect. 

Rosin, or Colophony. Rosin is the residue remaining in the still 
in the separation of oil of turpentine from crude turpentine. Its 
principal constituent is abietic anhydride. Rosin is about 90% 
saponifiable, the remaining 10% consisting of rosin oil. It melts 
anywhere from 100 to 140C, specific gravity 1.08. Its color 
varies from water white, pale amber, to black, but only the 
lighter amber colors are used in rubber manufacture. It has very 
little softening power, but is exceedingly tacky, so that it can be 
used only in small amounts in cements, frictions, and varnishes. 

Rosin Oil. By the destructive distillation of rosin, we obtain, 
amongst other products, a reddish colored oil, commonly called 
rosin oil. Its boiling point is around 360C, or over, specific 
gravity 0.98-1.10; it usually contains 10% to 20% of rosin, which 
is saponifiable, but the remaining 80% to 90% is an unsaponifi- 
able hydrocarbon. It will be noticed that rosin always contains a 
small amount of rosin oil, and vice versa, hence, both substances 
give the same positive reaction in the Liebermann-Storch test. 5 

Rosin oil adds very little to the tackiness of the rubber, and is 
essentially a softener. It improves the waterproofing qualities 
of rubber. 8 Rosin oil is not used very extensively, especially in 

"The simplest way to make this test is to warm a few drops of the oil in 
1-2 cc. of acetic anhydride, cool, and to a few drops on a porcelain test plate, 
add a drop of sulfuric acid of sp. g. about 1.5. A reddish violet color indicates 
rosin or rosin oil. It is believed that the unsaponifiable portion is really respon- 
sible for the color, and when examining for rosin or rosin oil, the test may be 
made much more delicate by making it upon the unsaponifiable portion. Bur- 
gundy pitch. Venice turpentine and similar resins, give practically the same 
color, so that the identification as rosin or rosin oil is not absolutely positive. 

• Rubber compounds are so frequently used for waterproofing and in such 
articles as rubber tubing, hot water bags, etc., that one is quite likely to over- 
look the fact that rubber takes up a large amount of water when left in contact 
with it for any length of time, and this holds true even after the rubber has 
been vulcanized. Pure gum sheet, vulcanized, has been found to absorb as 
much as 20% of its weight in water. C. O. Weber, in his book on India 



36 THE ANALYSIS OF RUBBER 

high grade goods, since it is a solvent, or rather a depolymerizer 
of rubber. The connection between the two substances, rosin oil 
and rubber, can readily be seen in the fact that crude turpentine 
is composed largely of the terpenes sylvestrene and australene, 
the composition of which is C 10 H 16 ; which form tetrabromides, 
ozonides, and polymerize easily. 

Tar Oils. The tar oils are the residues from the destructive 
distillation of wood or coal, the coal tars being the ones gener- 
ally used. They are of varying composition, and act merely as 
softeners. As a rule, they are soluble in acetone and alcohol, and 
have a specific gravity of about 1.00. Their properties depend 
largely upon the source of the crude material, and the degree of 
rectification. 

Glue. The glue used in rubber compounding is the ordinary 
granulated bone glue. The moisture content varies between 7% 
and 12%, and the specific gravity is about 1.25. Just as it comes, 
it may be mixed directly with rubber on a fairly warm mill. It 
is best to have the mixture refined while it is still hot in order 
to thoroughly break up any particles of glue. Several other 
methods are in vogue; 3 parts of glue are heated with 1 part of 
water until a smooth mixture is obtained, 7 then cooled until it 
sets to a firm jelly. This is mixed with rubber, and dried in a 
vacuum dryer. In other preparations, the glue is melted and 
mixed with oils or glycerin, and then allowed to cool; or it may 
be dissolved in water, gas black or other fillers stirred in, the 
solution concentrated, and the cooled mass mixed with rubber. 

The effect of glue on rubber is to reduce the elongation, and 
increase the permanent set. In many compounds, it has been 
found to exert a stabilizing effect on the cure, flattening out the 

Rubber and its Analysis, p. 14, says : "The water absorption of vulcanized 
rubber is extremely small, certainly not large enough to appreciably affect the 
insulation of a rubber cable after 5 years' continuous immersion." Weber did 
not state what kind of a compound he had in mind when he made this state- 
ment, but we have had experience with 40% fine Para compounds, containing 
about 2% of paraffin, which became absolutely waterlogged after about two 
or three years continuous immersion in water, and were utterly unfit for their 
purpose. To secure the best waterproofing properties, we resort to the addition 
of oils, waxes, and pitches. This is particularly true in electrical supplies. 

7 At this stage, several possibilities are open. Some add formaldehyde in 
sufficient quantities to produce an insoluble glue. Others have added glycerin, 
about 5% of the dry glue, and concentrated the solution until the moisture con- 
tent is from 15% to 20%. The purpose of this is to prevent the glue, on 
cooling, becoming hard and brittle. This glue-glycerin-water combination mixes 
readily with rubber, and in so doing, the moisture content is substantially 
reduced. 



THE PREPARATION OF RUBBER COMPOUNDS 37 

peak of the vulcanization curves, and reducing the danger of 
either over or under cures. It has a special field in rubber tubing 
for conducting gasoline, and other organic solvents, reducing 
greatly the effect of such solvents on the rubber. Glue has a 
slight accelerating effect on the vulcanization. 

Other Organic Fillers. A large number of organic substances 
are used in special articles, by reason of the real or fancied im- 
provement in the quality or service, from such addition, or for 
reasons of economy. Rubber soles may be stiffened with ground 
cotton fabric; shellac, hard gums and resins are used in cements 
and in waterproofing; ground cork or leather in some floor cover- 
ings, etc., etc. 

Vulcanizing Materials. 

Sulfur. The sulfur used in rubber should be dry, and free from 
acid, sand, or other impurities. Before using, it should be care- 
fully sifted through a 50 mesh screen, excepting, of course, in low 
grade compounds, where such refinements are of no value. The 
purpose of the sifting is to remove dirt, splinters of wood, etc., 
that may come from the container, and to remove agglomerations 
or lumps of sulfur. 

Tests for Sulfur. 

Acidity. Ten gr. of the sample is placed in a flask, with 
100 cc. of distilled water, heated on the water bath for 15-30 
minutes, and any acidity titrated with N/10 sodium carbonate, 
using methyl orange as the indicator. A blank is run on the 
water used. Not over 2-3 drops should be required to make the 
solution alkaline. 

Moisture. Dry at 85C for one hour in a neutral gas, 1 gr. 
of sample, cool and weigh. The loss should be negligible. 

Ash. Ignite 1 gr. of sulfur in a porcelain crucible, performing 
the burning in a hood with a strong draft. Cool the crucible and 
weigh. The ash should be less than 1 mg. 

Sulfur Chloride. The sulfur chloride used in rubber manufac- 
ture is the monochloride, S 2 C1 2 . Since, however, chlorine acts on 
the monochloride to give the dichloride, there is usually some of 
the latter present in commercial sulfur monochloride. Pure 
sulfur monochloride has a specific gravity of 1.709, boils at 138C, 
fumes strongly in the air, is decomposed by water forming sulfur 



38 THE ANALYSIS OF RUBBER 

dioxide, sulfur and hydrochloric acid. The sulfur liberated by 
the reaction with water is readily dissolved by the sulfur chloride. 
It is usually a red or a deep orange color. The dichloride, SC1 2 
has a specific gravity of 1.62, boils at 64C, and at the boiling 
point partially decomposes into S 2 C1 2 and Cl 2 . 

The commercial sulfur monochloride usually has a gravity be- 
tween 1.65 and 1.70, and a boiling point between 115C and 130C. 

Sulfur monochloride should be stored in a cool, dry spot, in 
clean earthenware jugs with tight fitting earthenware stoppers. 
It should not be exposed to the air, on account of its affinity 
for water. 8 

Organic Accelerators. 

The number of organic substances which accelerate the vul- 
canization of rubber is so great that we have deemed it quite 
unnecessary to attempt to deal with those which are only of 
casual interest. Primarily, we are dealing with the analysis of 
rubber goods, and are chiefly interested in the accelerators which 
are now being used commercially, or which show possibilities of 
becoming such. The most widely used organic accelerators today 
are aniline, thiocarbanilide, and hexamethylenetetramine, and 
the analyst should look first for these three before proceeding 
further. 

Most organic accelerators are used in small amounts. For 
very fast curing purposes, such as tire repair stocks, the quan- 
tity may be as high as 5% or 6% ; but for ordinary compounds 
the amount is usually 1% or less of the amount of rubber pres- 
ent. The amount used depends largely upon the time of cure 
desired, and the nature of the accelerator. 

Aniline. Aniline, or phenylamine (commonly called aniline 
oil) , is colorless when freshly distilled, but on standing, acquires 
a deep red color, and this is the condition in which it is found 
commercially. It is an oily liquid, specific gravity 1.02, boiling 
point 184.4C, melting point -6C. The melting point is a par- 
ticularly useful test for purity. 

• This reaction between sulfur monochloride and water will no doubt explain 
a considerable amount of the trouble experienced with acid splices, and acid cured 
goods in general, especially in the hot, sultry days in summer. The evaporation 
of the solvent of a cement cools the surface below the dew point, resulting in a 
deposit of a film of moisture. The latter reacts with the S 2 C1 2 , reducing the 
amount of the active vulcanizing substance which, in extreme cases, may be 
entirely destroyed before any vulcanization has taken place. 



THE PREPARATION OF RUBBER COMPOUNDS 39 

Hexamethylenetetr amine. A white crystalline powder, com- 
monly called hex, or hexa, melting point about 280C, but decom- 
poses below its melting point. Specific gravity 1.25. It is quite 
soluble in water, and slightly so in 95% alcohol. 

Thiocarbanilide. Thiocarbanilide, diphenylthiourea, CS 
(NHPh) 2 , commonly called thio, crystallizes in white plates, 
M.P. 154C, specific gravity 1.32. It is made by heating carbon 
bisulfide with aniline. The commercial product is usually a gray 
powder, and may contain small amounts of sulfur. There are 
at least a dozen trade names for this one accelerator, some of 
the preparations being a mixture of thio with inert pigments. 

Diphenylamine. Diphenylamine, or phenyl-aniline, NHPh 2 , 
has a molecular weight of 169, specific gravity 1.16, melting point 
54C, boiling point 302C. It is only slightly soluble in water. 

Dimethylaniline. Dimethylaniline, PhNMe 2 , is a yellow liquid, 
specific gravity 0.958, melting point 2.5C, boiling point 194C. It 
is very slightly soluble in water. 

Aldehyde Aniline. If well cooled formaldehyde is mixed with 
aniline, anhydroformaldehyde-aniline (or trimethylenetrianiline) 
is formed, melting point 140C. In alkaline solution, at ordinary 
temperatures, formaldehyde and aniline give methylene-diphenyl- 
diamine, CH 2 (NHPh) 2 , melting at 65C. This may also be pre- 
pared by heating anhydroformaldehyde-aniline with alcoholic 
aniline to 100C. 

Commercial aldehyde-aniline is a mixture of several sub- 
stances, the proportions varying with the differences in the con- 
trol during the process of manufacture. 

Ethylidene Aniline. Ethylidene aniline is made from acetalde- 
hyde and aniline. It is a dark reddish liquid, very stiff at 
ordinary temperatures, but it becomes quite fluid at the usual 
working temperatures of the mixing mill (175F-200F). 

P-nitrosodimethylaniline. P-nitrosodimethylaniline is obtained 
in the form of large green, glistening leaflets, melting point 85C. 
It stains paper or cotton a deep yellow. With caustic alkali, it 
breaks down into nitrosophenol and dimethylamine, a reaction 
of much interest in connection with the preparation of the dithio- 
carbamates. 

Other Aniline Derivatives. There are some other derivatives of 
aniline which might be included here, but are not because they 



2) 



40 THE ANALYSIS OF RUBBER 

are of no importance commercially. We may mention p-pheny 
lenediamine, p-aminodimethylaniline, etc. 

Diphenylguanidine. Diphenylguanidine, NH:C:(NHPh) 
melting point 147C. It is a mono-acid base; with carbon bisul- 
fide, it forms thiocarbanilide and thiocyanic acid. One com- 
mercial preparation consists of two thirds diphenylguanidine, and 
one third magnesium oxide. 

Triphenylguanidine. Two triphenylguanidines are known; 
(a) PhN:C: (NHPh) 2 , is most easily prepared by heating thio- 
carbanilide and aniline, and distilling off the excess of aniline. 
Hydrogen sulfide splits off during the reaction. This is the tri- 
phenylguanidine commonly used in rubber compounding. When 
pure, it exists as white crystals, but the commercial product is 
frequently colored yellow owing to the excess of aniline which 
has not been distilled. It has a melting point of 143C. (6) The 
second triphenylguanidine is derived from the HC1 salt of 
diphenylamine and cyananilide, the formula being NH:C. 
(PhNH).(Ph 2 N). It also has accelerating properties. 

Diphenylcarboimide. Diphenylcarboimide, C 13 H 10 N 2 ; if tri- 
phenylguanidine is heated under reduced pressure, aniline is given 
off and diphenylcarboimide produced, PhN:C:NPh. The crude 
substance is glassy, resinous, amorphous, with no definite melting 
point, but softens gradually as it is heated. The pure substance 
is said to have a melting point of 160C-170C. 

Aldehyde Ammonia. When formaldehyde combines with am- 
monia, instead of following the usual procedure, we get hexa- 
methylenetetramine. Aldehyde ammonia is the product of the 
combination of acetaldehyde and ammonia; Me.CHOH.NH 2 ; 
melting point 70C-80C, boiling point 100C. It occurs as color- 
less crystals, turning dark on exposure to the air; probably on 
account of the reaction with the moisture in the air, since in 
contact with water it forms hydroacetamide. 9 

Furfnramide. Furfuramide, formed by the action of ammonia 
on furfuraldehyde; a light brown crystalline substance, melting 
point 117C. 

Quinoidine. The product sold commercially under the name 
quinoidine, is the residue remaining after the removal of the 
alkaloids quinine, cinchonine, and cinchonidine, from the extract 
of Peruvian bark. It is a dark brown to black resinous solid, 

» Richter's Organic Chemistry, translation by E. F. Smith, II, p. 206. 



THE PREPARATION OF RUBBER COMPOUNDS 41 

non-crystalline, which softens readily, and mixes well with 
rubber. 

Piperidine. Piperidine is a colorless liquid, with a peculiar 
odor slightly resembling that of pepper; strongly basic, soluble 
in alcohol and water; boiling point 106C. It is found in nature 
in combination with piperic acid, as the alkaloid piperine, or 
piperyl-piperidine, crystallizing in prisms, melting point 129C. 
Piperine is chiefly of interest in combination with carbon disulfide, 
when it forms one of the ultra-rapid accelerators (see following). 

The So-Called "Ultra-rapid" Accelerators. The combination 
of carbon bisulfide with secondary amines such as dimethylamine, 
piperidine, piperine, pyrrolidine, etc., gives rise to the formation 
of substances which are extremely powerful accelerators of vul- 
canization; these are believed to be salts of dithiocarbamic acid, 
and the accelerators of this class are usually called the thiocar- 
bamates. They are so much more powerful than the organic 
accelerators that some have attempted to distinguish them by 
the name of "ultra-rapid accelerators." 10 

The dithiocarbamates are mono-basic, and with zinc form salts 
which form a second class of rapid accelerators. 

A third class of rapid accelerators, the thiurams, is formed by 
the oxidation of the dithiocarbamates ; the product is a derivative 
of thiuramdisulfide, NH 2 C-S-S-S-S-CNH 2 ; for example, the 
tetraethyl derivative would be (CSNEt 2 ) 2 S 2 , a white crystalline 
substance, with a melting point of 70C. A few of these product? 
may be mentioned as follows: 

Dimethylamine and carbon bisulfide; C 5 H 14 N 2 S 2 , m.p. 103C. 

Diethylamine and carbon bisulfide, C 9 H 22 N 2 S 2 ; m.p. 130C. 

Thiuramdisulfide; NH 2 CS.S.S.SC.NH 2 . 

Tetramethylthiuram disulfide, (CSNMe 2 ) 2 S 2 ; m.p. . 

Tetraethylthiuram disulfide, (CSNEt 2 ) 2 S 2 ; m.p. 70C. 

The above list includes practically all of the organic accelera- 
tors which have reached any commercial significance, and per- 
haps a few that have not as yet. There is still the derivatives of 
quinoline, pyrrole, piperidine, and many others. In fact, it may 

10 Some idea of their power to accelerate vulcanization may be gleaned from 
the fact that a mixture of 50 parts each of rubber and zinc oxide, 3 parts of 
sulfur, and only 0.1 part of the dimethyldithiocarbamate, will reach its maxi- 
mum cure in three minutes. Some of the others in this class are even more 
rapid in this, giving good cures in one minute, with slabs about one sixteenth 
of an inch thick, hardly time enough for the heat to penetrate to the center 
of the sheet. 



42 THE ANALYSIS OF RUBBER 

not be going too far to say that any basic organic compound, 
containing amino, or imino nitrogen, is a promising substance in 
which to look for accelerating properties. 

Inorganic Accelerators. 

The inorganic accelerators are practically limited to com- 
pounds of two elements, lead and magnesium. Calcium hydrox- 
ide has accelerating power, but it can be used in such small 
quantities, on account of its hardening effect on a compound, 
that sufficient of it cannot be used to completely accelerate the 
cure. Sodium hydroxide in small amounts acts as an accelerator, 
while in amounts in the neighborhood of 5%, it actually retards 
vulcanization. The lead compounds are litharge, red lead, basic 
lead carbonate, sublimed white lead, sublimed blue lead, and 
lead oleate. Magnesium oxide and carbonate are the only mag- 
nesium compounds. 

Litharge. 

Litharge should be clean, dry, pale yellow in color, free from 
copper; specific gravity 9.37. There should be only small 
amounts of the dioxide. Litharge is used in quantities of from 
5% to 20%. Of special interest is the manufacture of aprons 
for the protection of workers with radio-active substances. These 
contain about 90% of litharge, 9% of rubber, and 1% of sulfur, 
by weight. 

Tests for Litharge. 

Moisture. Dry 2 gr. of the sample at 105C for 2 hours, cool 
and weigh. 

Lead Dioodde. 11 Treat 1 gr. of the sample in a beaker 
with 15 cc. of nitric acid, sp.g 1.20. Stir the sample until all 
trace of red color has disappeared. Add from a calibrated pipette 
or burette exactly 10 cc. of dilute hydrogen peroxide (1 part 
of 3% hydrogen peroxide to 3.5 parts of water). Add about 
50 cc. of hot water, and stir until all of the lead dioxide has passed 
into solution. In the case of some coarsely ground oxides, the 
contents of the beaker may have to be heated gently to effect 
complete solution. After the oxide has gone into solution com- 
pletely, dilute with hot water to 250 cc, titrate with potassium 

11 The Chemical Analysis of Lead and its Compounds, by John A. Scbaeffer 
and Bernard S. White, pub. by Picher Lead Co.. Joplin, Mo. 



THE PREPARATION OF RUBBER COMPOUNDS 43 

permanganate solution having an iron value of about .005. Run 
a blank on the hydrogen peroxide. 

If the permanganate has been standardized in terms of iron, 
it can be calculated to lead dioxide, using the factor 2.134. From 
this the total weight of the dioxide can be calculated. 

Copper. Dissolve 20 gr. of litharge in dilute nitric acid, and 
boil until solution is complete. Add 40 cc. dilute sulfuric acid, 
boil gently for one hour, and allow to cool. Filter off the lead 
sulphate and wash thoroughly. Nearly neutralize the acid with 
ammonia, make acid with hydrochloric acid, warm the solution, 
and pass in hydrogen sulfide. Filter the precipitate, without 
washing, using some of the filtrate to transfer the last traces of 
sulfide to the paper. Dissolve in nitric acid, and wash the paper 
thoroughly with hot water. Add 3 cc. of cone, sulfuric acid, 
evaporate until the fumes of sulfuric acid are evolved, cool, 
dilute, and, after standing, filter again, washing with hot water 
containing a little sulfuric acid. Precipitate the copper in the 
filtrate as sulfide in an ammoniacal solution, filter, ignite and 
weigh in a covered porcelain crucible. The residue will be a mix- 
ture of CuO and Cu 2 S. Since the percentage of copper is the 
same in both cases, calculate to copper using the factor 0.7988. 

Fineness. Determine the residue on a 200 mesh screen, using 
water to wash the pigment through, and breaking up any loose 
lumps with a rubber policeman. 

Red Lead. 

Red lead is a mixture of the monoxide and dioxide, with a 
specific gravity of 9.07. It should have a bright red color, be 
clean and dry. The moisture, lead dioxide, copper and fineness 
may be determined as under litharge. 

White Lead. 

White lead is the basic carbonate, containing about 80% 
metallic lead, and 20% of carbon dioxide and combined water. 
The specific gravity is 6.46. 

Tests for White Lead. 

Total Lead. 12 Weigh 1 gr. of the sample, moisten with water, 
dissolve in acetic acid, and filter, ignite and weigh the impurities. 
Add to the filtrate 25 cc sulfuric acid (1-1), evaporate until 

" P. H. Walker, Bull. 109, Bureau of Chemistry, U. S. Dept. of Agriculture. 



44 THE ANALYSIS OF RUBBER 

the acetic acid is driven off; cool and dilute to 200 cc. with water, 
add 20 cc. ethyl alcohol, allow to stand for 2 hours, filter on 
a Gooch crucible, wash with 1% sulfuric acid, ignite and weigh 
as lead sulfate. Calculate to lead with the factor 0.6829 or to 
the basic carbonate by 0.8526. 

Carbonic Acid. A 1 gr. sample is placed in a flask containing 
a side arm delivery tube connected with a train consisting of 
two U-tubes containing sulfuric acid and potassium bichromate, 
two U-tubes containing soda-lime, and the fifth U-tube contain- 
ing the same solution as the second sulfuric-bichromate tube. 
Add dilute nitric acid, and sweep out the liberated carbon dioxide 
with a current of air which has been freed from carbon dioxide 
by passing over soda-lime. Weigh the two soda-lime tubes, 
and the fifth tube, containing sulfuric acid-bichromate; the in- 
crease in weight is carbon dioxide. 

Fineness. Treat as under litharge. 

Sublimed White Lead. 

Commercial sublimed white lead is a basic sulfate, containing, 
on an average, of about 78.5% of lead sulfate, 16% of lead oxide, 
and 5.5% of zinc oxide. It has a specific gravity of 6.20. It 
should pass through a 200 mesh screen without appreciable 
residue. • 

Sublimed white lead is used for its accelerating properties, 
which are almost entirely dependent upon the content of lead 
oxide. A test mix would undoubtedly be the best method for 
testing; the lead oxide may be calculated by determining the 
total sulfur and total lead, and after calculating the sulfur to 
lead sulfate the excess of lead may be calculated to lead oxide. 

Sublimed Blue Lead. 

Sublimed blue lead contains lead sulfate, sulfide, sulfite, oxide, 
and zinc oxide, with occasional traces of carbon. The fineness 
and accelerating properties are the only elements of interest; 
the specific gravity will be about 6.50 to 7.0. 

Lead Oleate. 

Lead oleate is a yellowish soft waxy solid, used to replace 
litharge because of the ease with which it may be distributed in 
a rubber mixing. The specific gravity is 1.50. It is claimed that 



THE PREPARATION OF RUBBER COMPOUNDS 45 

the lead oleate is much less harsh in its action than litharge, with 
less danger of burning the stock. 

Magnesium Oxide. 

Magnesium oxide, MgO, is sometimes called calcined magnesia 
from its method of preparation; it exerts a considerable influ- 
ence on the vulcanization of rubber, although less than that of 
litharge. It is prepared by precipitation as the carbonate, and 
the latter ignited. It usually contains some calcium carbonate, 
but the amount must be kept very low in order not to interfere 
with its accelerating power. It has a specific gravity of from 3.20 
to 3.45. 

The calcium carbonate may be determined by solution of the 
sample in hydrochloric acid, and the separation of the calcium 
as oxalate from an ammoniacal solution, with ammonium 
oxalate. The calcium may then be determined in any desired 
way. 

Because of its effect on the action of certain organic accel- 
erators, magnesium oxide is sometimes used in amounts of 0.25% 
to 1.0%, in which case the accelerating effect of the magnesium 
oxide so used is small compared with that of the activated or- 
ganic accelerator. As the principal, if not the only accelerator, 
it will be found in amounts up to 10%. 

Magnesium Carbonate. Magnesium carbonate is a light, white 
powder, existing in a finer state of division than the oxide; its 
specific gravity is around 2.22. It may also contain calcium 
carbonate, which may be determined as under magnesium oxide. 

The carbonate is not as powerful an accelerator as the oxide, 
and hence will be found in somewhat larger amounts; it is sel- 
dom used in less than amounts around 5%, and may go as high 
as 20%. 

In the absence of any appreciable amounts of calcium, deter- 
mine the magnesia content of the dry pigment by igniting to a 
dull red heat, to constant weight, taking care that the residue is 
cooled in a desiccator, and weighed in a stoppered weighing 
bottle, in order to prevent reabsorption of moisture. 

Inorganic Fillers. 

Aluminum Flake. Aluminum flake is essentially a mixture of 
hydrated aluminium oxide and silicate. It is a white powder, 



46 THE ANALYSIS OF RUBBER 

with a specific gravity of from 2.58 to 2.65; with 2.60 as a fair 
average of the commercial lots. It contains very little moisture 
which may be driven off by heating at 100C. Continued ignition 
at a dull red heat shows an ignition loss of about 12% ; the resi- 
due is the oxide and silicate. The ignited oxide is difficult to 
get into solution in hydrochloric acid, even when fused for 
a short time with sodium carbonate. This fact is important, 
both in the examination of the pigment, and in the analyses of 
ash. 

On account of its low gravity and fineness, it is used to replace 
some of the zinc oxide in a compound, although it does not give 
as good tensile properties. 

Ammonium Carbonate. 

Commercial ammonium carbonate is a mixture of the car- 
bonate and carbamate; it is used to supply the gas for making 
sponge rubbers. 

Asbestine. 

Asbestine is the trade name for a fairly pure magnesium sili- 
cate, specific gravity 2.60-2.80. It is used at times in place of 
talc for dusting stocks, and replaces whiting in some mixes. It 
is a cheap filling material. 

Barytes. 

Barium sulfate is used under various trade names, barytes, 
blanc fixe, basofor, barium dust, etc. Wiegand has shown that 
this pigment is a mere diluent; it is inert during vulcanization. 
On account of the crystalline nature of this pigment, it is not 
very well adapted for some lines of manufacture, but finds ex- 
tensive use in mechanical goods. The specific gravity runs be- 
tween 4.2 and 4.5. It should be free from grit and should leave 
no residue on a 200 mesh screen. Some preparations of barytes 
are claimed to have less than 1 % of residue on a 300 mesh screen. 
The best means for telling the relative value of the various 
brands of barytes is by means of vulcanization tests with experi- 
mental batches. 

Since barytes is used merely as a filler, it is seldom found in 
amounts under 10%, and there may be as high as 30% in the 
compound. 



THE PREPARATION OF RUBBER COMPOUNDS 47 

Brown Pigments. 

The principal brown pigments are the various mixtures of 
iron and manganese oxides, the umbers. These are usually higher 
in manganese than the siennas. They should be tested for grit, 
and for change of color when heated. 

Recent research has seemed to indicate that manganese is re- 
sponsible for rapid deterioration of some rubber compounds; 
should this be substantiated with further work, it would seem 
to show that the manganese browns should be used with caution. 

Calcium Sulfate. 

Calcium sulfate is rarely used as such in rubber compounding, 
but it exists as a part of many lots of commercial golden and 
crimson sulfides of antimony. 

Chinese Blue. 

Blue is not a color which is used to any very great extent in 
rubber manufacture. The chief blues are Chinese blue, ultra- 
marine blue, and the blue organic dyes. 

Chinese (or Prussian) blue, is precipitated from a mixture of 
potassium ferrocyanide and ferric sulfate. It is an excellent blue 
color, but has limited possibilities in rubber, owing to its turning 
brown when mixed with alkalies, forming ferric oxide, and salts 
of hydrocyanic acid. 

Crimson Antimony. 

Crimson antimony is largely an oxide or oxysulfide of anti- 
mony, with a deep crimson, or red color; specific gravity varies 
from 3.9 to 4.2. It is usually lower in free sulfur than golden 
sulfide, and is used chiefly on account of its color. 

Dyes. 

The organic dyes are found chiefly in the sulfur chloride, or 
acid, cured goods. Practically none of them are water soluble, 
but most of them can be leached out with alcohol, acetone, or 
benzene. The identification of these dyes is an exceedingly diffi- 



48 THE ANALYSIS OF RUBBER 

cult proposition; they are, as a rule, merely coloring materials, 
and have no other effect on the rubber, so that any dye which 
will give the same color is no doubt of equal value, and the 
positive identification of any one particular dye is not often a 
matter of interest. 

Fossil Flour. 

Fossil flour (tripoli, diatomaceous earth) consists of the re- 
mains of diatoms, and is nearly pure silica, with traces of alkali. 
It may contain considerable moisture, and the loss in weight at 
105C is an important indication of its availability for rubber 
compounding. It is a very poor conductor of heat, and hence 
is frequently used in steam valves, etc. The specific gravity is 
about 2.00. 

Gas Black. 

Gas black is a very pure form of carbon, prepared by burning 
natural gas with insufficient air for complete combustion. It is 
the most finely divided pigment in use in rubber compounding; 
it contains no oil or grease, and on ignition leaves no residue. 
It has a specific gravity of 1.73, or less than one third of that 
of zinc oxide, so that a pound of gas black has more than three 
times the volume, and an even greater proportion of active sur- 
face. It is hygroscopic to a considerable degree, taking up mois- 
ture from the air to the extent of 2 or 3%. 

Gas black should not be confused with lamp black, which is 
made from the burning of oils, tars, or resins, also with insuf- 
ficient air for complete combustion. The flame may impinge 
on a revolving metallic cylinder, as in the case of gas black, or 
the oil may be fired in a huge oven, and the smoke carried 
through a series of chambers, thus making a partial separation 
of the different grades of black. Those nearest the fire are, of 
course, heavier, and contain a larger percentage of oil than the 
black contained in those chambers furthest away from the fire. 
These lamp blacks are further purified by heating, with the ex- 
clusion of air, thus reducing the percentage of oil. Lamp black 
is not as fine a pigment as gas black, and does not give the same 
improvement in tensile properties that the latter does; in fact, 
in this respect, it is rated below zinc oxide. It has the same 
specific gravity as gas black. 



THE PREPARATION OF RUBBER COMPOUNDS 49 

The only tests for gas black, or lamp black, are moisture, oils, 
and ash. Moisture should be determined on a 1 gr. sample by 
heating to 105C, cooling in a desiccator, and weighing in a stop- 
pered weighing bottle. Oil is determined by extraction of a 5 gr. 
sample with ethyl ether, and weighing the residue. Not less 
than 5 gr. should be taken for the ash, and if the residue is an 
appreciable amount, it shows an admixture of other blacks, or 
dirt. 

Owing to its low gravity, and fineness of particle size, gas 
black seldom runs higher than 10%, although there have been 
commercial articles manufactured containing 17-20%. 

Golden Antimony. 

Antimony sulfide, or golden sulfide, is a mixture of the tri- and 
penta-sulfides of antimony, free sulfur, and it may contain little 
or much calcium sulfate. The pigment varies from orange to a 
reddish color, the red being due to the oxide or oxysulfide. The 
composition varies within wide limits, as is shown by the varia- 
tion in the specific gravity of from 2.5 to 2.9. It is not an accel- 
erator of vulcanization; its real value consists in its ability to 
give up the free sulfur to rubber during vulcanization and yet, 
afterwards, to remain free from blooming. The free sulfur 
should run about 17%, and the calcium sulfate should be low. 
Caspari gives some figures showing that the free sulfur may vary 
from 7 to 19% ; the calcium sulfate from 3 to 50% ; and the anti- 
mony sulfides from 30 to 90%. 

When used for coloring only, golden sulfide may be used only 
to the extent of 1 or 2% ; when used as the source of sulfur for 
vulcanization, 15 to 25% will be required, depending largely 
upon the free sulfur and antimony sulfide content of the dry 
pigment. 

Tests for Golden Sulfide. 

Calcium sulfate. Jacobson 13 recommends the following simple 
test for calcium sulfate: Mix 1 gr. of the original sample 
with 2 gr. of sublimed ammonium sulfate in a porcelain crucible. 
Heat until the ammonium sulfate and antimony sulfide have been 
driven off; cool and weigh. 

"Chem. Ztg. 32, 984 (1908). 



50 THE ANALYSIS OF RUBBER 

Free sulfur. Extract 1 gr. with acetone, or carbon bisulfide, in 
the extractor described under "acetone extract." Distil off the 
solvent, add 100 cc. of water and 3 to 5 cc. of bromine; proceed 
with the determination as directed under the determination of 
free sulfur in vulcanized articles. Or the solvent may be driven 
off in a tared flask, the flask and contents dried to constant 
weight at 90C, and the sulfur weighed directly. This method is 
shorter, but as a rule, not as accurate. 14 

Graphite. 

Graphite, or plumbago, is a natural form of carbon, used to 
some extent on account of its lubricating value in preventing 
adhesion between rubber stocks and metal. It may be found in 
some stocks where its acid and alkali resisting properties are 
of peculiar value. 

Greens. 

Most of the green pigments used in rubber manufacture are 
organic colors. Brunswick green, a mixture of Chinese blue and 
chrome yellow (lead chromate), darkens when heated with sul- 
fur. This green is sometimes marketed as "chrome green," but 
the true chrome green is the oxide of chromium, Cr 2 3 , and is by 
far the best mineral green for rubber work, since it is not readily 
affected by heat, acids, or alkalies. 

"Luff and Porritt, J. Soc. Chem. Ind. Ifi, 275-8T (1921), found by previously 
heating antimony sulfide before extracting the free sulfur, the latter varied 
considerably, as will be seen from the following table: 

Sulfur Extracted from Antimony Sulfide. 
Extraction for 5 Hours with Carbon Bisulfide. 





Unheated 






Heated 1 to 2 hours 






1st 5 


2nd 5 








Sample 


hours 


hours 


125 C 


150 C 


230 C 


1 


3.70 


0.33 


2.99 


4.88 


6.94 


2 


31.21 


0.33 


29.75 


32.19 


32.71 


3 


1.02 


0.13 


.95 


1.01 


.98 


4 


4.64 


0.17 


1.56 


4.86 


4.90 


5 


9.14 


0.13 


8.90 


13.74 


15.38 



The presumption is that the sulfur extracted is available for vulcanization. 
If during vulcanization a greater percentage of free sulfur than that indicated 
at normal temperatures is available, this fact is of decided interest and value. 
It is desirable that this subject be followed up — we should know more definitely 
why at 125C the free sulfur drops off, and more particularly how long, after 
heating, the additional free sulfur is capable of being extracted with carbon 
bisulfide. 



THE PREPARATION OF RUBBER COMPOUNDS 51 

Iron Oxides. 

Red oxide of iron (Indian red, Venetian red) is one of our 
most valuable pigments, not merely for its color, but for the 
valuable tensile properties which it imparts to rubber, ranking, 
in this respect, not very far behind zinc oxide. It is practically 
pure Fe 2 3 , running over 98%, with small amounts of water. It 
holds its color very well during vulcanization. The specific 
gravity is between 5.0 and 5.20. 

These iron oxides may be obtained in a great variety of shades, 
depending largely on the method of preparation. The color 
should always be matched against a standard, and it is best to 
make a heat test at 150C, as recommended for golden sulfide. 

Lime. 

The lime which we use is the air slaked hydroxide, specific 
gravity of 2.4. It is used largely because it will take up small 
amounts of moisture which may be present in the compound, 
and reduce the danger of ''blowing," or porosity. It has a decided 
hardening effect on the rubber, and hence may not be used in 
anything but small amounts. It also is believed to be responsible 
for rapid deterioration. It has some accelerating effect on the 
vulcanization, and due allowance must be made for this factor. 

Lithopone. 

Lithopone is a mixture of barium sulfate and zinc sulfide, con- 
taining about 25 to 30% of the latter. It is not as fine a pigment 
as the oxide, and does not produce as good tensile properties. It 
is unaltered during vulcanization, and is often used as a substi- 
tute for the more expensive zinc oxide. It must be low in water 
soluble matter, lead, and chlorides. The specific gravity is 4.20. 

Tests for Lithopone. 

Moisture. Heat 1 gr. for 2 hours at 105C, cool and weigh. 

Barium Sulfate. To 1 gr. of pigment, add 10 cc. cone, hydro- 
chloric acid and 1 gr. of potassium chlorate in small portions. 
Evaporate to half its volume, add 100 cc. of hot water, and a 



52 THE ANALYSIS OF RUBBER 

few cc. of dilute sulfuric acid. Boil and filter, wash thoroughly, 
ignite and weigh the barium sulfate. Any silica, and some of 
the alumina, if present, would be included, but it is not worth 
attempting to make a separation. 

Total zinc. 15 Take 1 gr. of the pigment, and boil with the 
following solution: Water 30 cc, ammonium chloride 4 grams, 
cone, hydrochloric acid 6 cc. Dilute to 200 cc. with hot water; 
add 2 cc. of a saturated solution of sodium thiosulfate, and titrate 
with a standard solution of potassium ferrocyanide, using 5% 
uranium nitrate as an outside indicator. Calculate the zinc to 
zinc sulfide. 16 

Fineness. Lithopone should leave practically no residue on a 
200 mesh screen. 

Sodium Bicarbonate. 

Sodium bicarbonate is used in sponge rubber, since on heating 
it breaks down into the carbonate, carbon dioxide, and water. 
In the vulcanized article, it is found chiefly as the carbonate, 
Na 2 C0 3 . 

Talc. 

Talc is used extensively as a lubricant, to prevent rubber sur- 
faces from sticking together, and in molds, to prevent the rubber 
stocks from sticking to the mold. It is rarely used as a filler, 
but rubber has such a facility for absorbing talc that the analyst 
will rarely fail to find 1% or 2% of talc in vulcanized compounds. 
The specific gravity is about 2.7, and the color will vary from a 
brilliant white to a dirty gray. 

Talc usually has a considerable amount of grit, largely sand 
and the iron minerals which are usually found associated with 
talc (pyroxene, hornblende and biotite). 

Ultramarine. 

Ultramarine is probably a double silicate of sodium and 
aluminium, with some sodium sulfide. The sulfide seems an 
essential part; at least, if treated with acids, hydrogen sulfide is 
given off and the blue color fades out. It is the best known blue 

10 Low's Technical Methods of Ore Analysis, p. 284. 

i« There Is a slight error here, owing to the fact that part of the zinc is 
present as the oxide, but the error is usually negligible. 



THE PREPARATION OF RUBBER COMPOUNDS 53 

pigment for hot vulcanization, but it is not safe to use it in goods 
for acid curing, since sulfur chloride usually contains free acid, 
and the latter would react with the ultramarine, and either par- 
tially or wholly destroy the color. The specific gravity is 2.35. 

Vermilion. 

The true vermilion is the sulfide of mercury, a very heavy 
pigment, specific gravity of about 8.00, but possessing a brilliant 
red color. It is the most expensive pigment used in commercial 
rubber goods, and since its color is its only good point, it is sel- 
dom worth what it costs, and is not likely to be encountered by 
the average analyst. Some so-called vermilions are merely red 
lakes. In the dry pigment, they are easily recognized by the 
difference in gravity. 

Whiting. 

On account of its low cost, whiting is extensively used. It is 
essentially calcium carbonate, and should be entirely soluble in 
dilute acids, and should contain no free alkali. It is somewhat 
hygroscopic, specific gravity 2.67, and contains small amounts 
of iron, alumina, and silica. It may be found in any amount up 
to say 25 or 30%. 

Tests for Whiting. 

Moisture. Heat 2 gr. for 2 hours at 105C; cool and weigh. 

Free Alkali. In an Erlenmeyer flask, shake 10 gr. of pigment 
with 100 cc. of water, add a few drops of phenolphthalein ; the 
color should not be deeper than a faint pink. 

Water Soluble. Heat 10 gr. of pigment with 100 cc. of distilled 
water, filter, evaporate to dryness in a weighed beaker or dish, 
heat to 105C for 15 minutes, cool and weigh. 

Fineness. Whiting should leave practically no residue on a 
200 mesh screen. 

Yellow Ochre. 

The yellow ochres are practically all clays, containing large 
amounts of hydrated iron oxide; the specific gravity will vary 
enormously, probably more than any other pigment, from say 
3.50 to 5.00. The higher gravity ochres are considered better 



54 THE ANALYSIS OF RUBBER 

for the purpose; they hold their color better, have a stronger 
color, and are less likely to change color during vulcanization. 
The stronger colored ochres are to be preferred also, because less 
is required to give a definite color in the finished article. 

Zinc Oxide. 

Zinc oxide is unquestionably the most widely used pigment 
in rubber manufacture. Its extreme fineness makes it particu- 
larly valuable where strength and wear-resisting qualities are 
desired, it is unaffected in color during vulcanization, and hence 
can be used in any color combination. It has a special field of 
usefulness in that it also provides a rubber mix with an alkaline 
reaction, which permits many of the organic accelerators to func- 
tion. Thiocarbanilide, the dithiocarbamates, thiurams, etc., will 
not accelerate vulcanization unless the mixture is basic, and zinc 
oxide answers the purpose in a most acceptable manner. 

With some accelerators, zinc oxide reacts during vulcaniza- 
tion to form a new accelerator. The mechanism of such reac- 
tions is still a matter under investigation, and while splendid 
results have been accomplished by the workers in this field, we 
can hardly feel that the last word has been said on the subject. 
Probably the safest position to take is to say that practically all 
of the organic accelerators are more active in the presence of a 
basic oxide, such as magnesium, zinc and lead, and there are 
some which will not react without some such basic substance. 
In a few cases the marked difference between the reaction when 
zinc oxide is present, compared with some other basic oxide, 
suggests a possible reaction between the zinc oxide and the ac- 
celerator. 

Zinc oxide may be absent altogether, it may constitute only 
a small percentage of the whole compound, or it may be as high 
as 50%, as for example, in some of the white tire treads. 

Tests for Zinc Oxide. 

Zinc oxide should be tested for moisture, lead, chlorides, sul- 
fates, sulfides, and water-soluble matter. The specific gravity 
is 5.57, and the fineness such that there should be no residue on 
a 200 mesh screen, and very little on a 300 mesh. Over 0.1% of 



THE PREPARATION OF RUBBER COMPOUNDS 55 

lead renders it unfit for bright colored mixes, while much larger 
amounts would so change the vulcanization as to prevent its use 
altogether, unless, which seems unlikely, one could depend upon 
getting a zinc oxide with absolutely constant lead content. 
Chlorine is seldom found in amounts over 0.01, but cases have 
been known in which the chlorine ran over 0.20%. Such an 
amount will usually be reflected in an unusually high water 
soluble extract. Metallic chlorides have a deleterious effect on 
many rubber compounds, especially cements, and hence the 
chlorine content must be kept low. 

Moisture. Dry 2 gr. at 105C for 2 hours, cool, weigh, and cal- 
culate the loss to percentage. 

Insoluble Matter. In a 250 cc. beaker, treat 10 gr. with 50 cc. 
of cone, hydrochloric acid; evaporate to dryness, take up the 
residue with water and a few drops of hydrochloric acid, filter, 
and wash thoroughly with hot water. Ignite the residue, cool 
and weigh. 

Water Soluble. Treat 10 gr. with 200 cc. of water, heat on a 
hot plate for one hour, filter into a 250 cc. graduated flask, cool 
to room temperature, and make up to the mark. Take a 50 cc. 
portion, evaporate to dryness in a weighed beaker or dish, dry 
for 2 hours at 105 C, cool and weigh. 

Chlorides. From the water soluble extract take a 50 cc. por- 
tion, make slightly acid with nitric acid, add 10 cc. N/10 silver 
nitrate and a few drops of ferric chloride; titrate the excess of 
silver nitrate with standard ammonium thiocyanate. 

Sulfates. To another 50 cc. portion of the soluble matter, add 
several drops of cone, hydrochloric acid, and heat; add 1 cc. of 
10% barium chloride solution, allow to stand overnight; the 
next day, if there is any precipitate, it can be determined as 
usual. 

Total Sulfur. Treat 10 gr. of pigment with 25 cc. of cone, 
hydrochloric acid and 10 cc. bromine water. Evaporate to 
dryness, take up with 50 cc. of hot water and a few drops of 
hydrochloric acid, filter from any insoluble matter; heat nearly 
to boiling, add 1 cc. of 10% barium chloride, and after standing 
overnight determine any barium sulfate which may have pre- 
cipitated in the usual manner. 

Lead. The filtrate from the determination of insoluble matter 
is nearly neutralized with sodium carbonate, and the lead pre- 



56 THE ANALYSIS OF RUBBER 

cipitated with hydrogen sulfide. The qualitative test is usually- 
sufficient; if desired, the lead sulfide may be determined by any 
of the usual methods. 

Fineness. Place 10 gr. on a 200 mesh screen sieve, and, with 
a gentle current of water, wash the pigment through the screen. 
Any loose aggregates may be broken up with a policeman. There 
should be no residue. Repeat with a 300 mesh screen, and if any- 
thing remains on the screen, it should be transferred to filter 
paper, ignited, and weighed. 



Chapter IV. 
The Theory of Vulcanization. 

Having before us the proposition that we are at this time pri- 
marily interested in the process of vulcanization as a change in 
chemical composition, without necessarily dealing with the man- 
ner in which such a change takes place, it seems as though a 
detailed study of the various theories of vulcanization is quite 
beyond the scope of the present work, and we will therefore go 
into the subject only to the extent necessary to develop the facts 
regarding the chemical changes during this process. 

The term vulcanization has been used freely, and it will no 
doubt clarify matters if we attempt to define it. 1 For our pur- 
pose, we will assume that vulcanization will mean the addition 
of any element or group of elements, of which we may use sulfur 
and sulfur chloride as the principal examples, to crude rubber, 
or a mixture of crude rubber with other substances, whereby 
the crude rubber, or rubber mixing, is changed from a sticky, 
plastic mass into a substance having a certain degree of tough- 
ness, hardness, resiliency, and, in general, such properties as are 
usually associated with what we know as vulcanized rubber. By 
this definition, we purposely avoid including time or temperature 

1 It is of peculiar interest that while this book was in press an article by 
"Andrew H. King" appeared in Chem. & Met. Eng. 25, 1038-42 (1921), on "The 
Aging of Rubber," in which he gave a definition of vulcanization which so 
nearly parallels our own, as to make it well worth while quoting what he has 
to say: 

"By vulcanization, we mean the addition of a substance or substances to 
rubber, which results in the production of a more elastic material, i.e., one with 
less plasticity. When the change becomes of a sufficient magnitude that the 
product becomes of commercial value, it is then known as 'cure.' It is well 
known that substances other than sulfur or sulfur chloride — for example, 
oxygen, chlorine, selenium, etc., produce a change in plasticity — in other words, 
they vulcanize — but the products obtained in this way have not to date had any 
commercial value, and therefore cannot be called 'cured.' In speaking of addi- 
tional vulcanization, it is to be understood that we are not limiting ourselves 
to sulfur or sulfur chloride." 

Later on, in the same article, "King" says : "A surface aging which results 
in hardening or checking of the surface, is probably due largely to additional 
vulcanization by oxygen ; — internal aging may be sulfur and oxygen." 

57 



58 THE ANALYSIS OF RUBBER 

as a definite factor in the process; nor do we say that the 
process can, or cannot take place in the presence or absence of 
substances which may act as catalysts. The point which we wish 
to make regarding catalysts is, that they change the reaction as 
regards time, or temperature, or perhaps both, but they do not 
change the principal reaction itself. Taking the reaction between 
sulfur and crude rubber as an example, we finally come to the 
point where the rubber is saturated with sulfur, and has the 
formula (C 10 H 16 S 2 ) X . By the use of catalysts, we would get 
exactly the same product, only in a shorter time, or at a lower 
temperature. The use of these catalysts is therefore a matter of 
commercial economy of time. It is true, when we use the longer 
processes, or higher temperatures, we have side reactions, depoly- 
merization, etc., but the main process is the same in each case. 

C. 0. Weber found that when he heated rubber and sulfur to- 
gether, he obtained a substance having as high as 32% of sulfur, 
corresponding to (C 10 H 16 S 2 ) X ; with sulfur chloride, he obtained 
(C 10 H 16 S 2 C1 2 ) X . He therefore decided that rubber was a poly- 
prene, and that it combined with sulfur to form a series of poly- 
prene sulfides, the final product being identified as above. He 
was unable to isolate any of the intermediate products, and was 
obliged to assume their existence. Ostwald, reviewing the work 
of Weber, Hohn, Seeligmann, Axelrod, Hubener, Stern, Hinrich- 
sen, and others, came to the conclusion that the chemical theory 
did not satisfactorily explain the matter, and that the facts as 
known were more in accordance with an adsorption process than 
a chemical one. He based his conclusions on the following: 
That there was always at least a small amount of free sulfur re- 
maining after vulcanization (but which we now know is not so) ; 
that the process was a reversible one and that the rate of adsorp- 
tion depended upon the amount of working which the rubber 
sustained. Special emphasis was laid on the temperature coeffi- 
cient, 1.87, which seemed to agree more with the coefficient for 
an adsorption process than a chemical one. Ostwald was perhaps 
correct in saying that the evidence at the time was not sufficient 
to sustain the contention that the process was a chemical one; 
on the other hand, he himself included facts which as Loewen 2 
has pointed out, are explainable only on the theory of a chemical 
process. In the preparation of the bromine and nitrosite deriva- 

*Z. Angew. Cbem. 25, 1553-60. 



THE THEORY OF VULCANIZATION 59 

tives of rubber, it has been observed that the derivatives carry 
all of the combined sulfur, which would seem to indicate a chemi- 
cal bond between the rubber and the sulfur. Spence showed that 
not only did the bromine derivatives carry all of the combined 
sulfur, but that in a series of compounds, the bromine and sulfur 
bore stoichiometric relations. Spence found evidence of an 
adsorption effect between the free sulfur and the rubber, preced- 
ing the actual chemical combination of the two. We shall see in 
due course, when taking up the subject of the direct determination 
of rubber, the importance of the conclusions which we reach 
regarding the nature of the reaction between rubber and sulfur. 

Having arrived at the conclusion that the reaction is a chemi- 
cal one, we may pass on to the mechanism of the reaction. Crude 
rubber will not combine with sulfur to any appreciable extent at 
ordinary temperatures. With the exception of what we have 
called the ultra-rapid accelerators (dithiocarbamates, etc.) there 
is no appreciable reaction until a temperature of at least 100C 
is obtained, and, for ordinary mixes, the rate at this temperature is 
exceedingly slow. The ordinary commercial range may be said to 
be between 125C and 150C. While exact data are lacking it has 
been estimated that for each 6 to 8C increase in temperature, the 
speed of the reaction is doubled, i.e., the time required for correct 
vulcanization is reduced by one half. 3 Furthermore, the speed of 
the reaction may be enormously altered by the addition of cat- 
alysts. It will be noted that the reaction takes place best when 
the mixture is weakly alkaline; acids, or strong alkalies, retard or 
even prevent the combination of rubber and sulfur. About 0.1% 
of caustic soda acts as an accelerating agent, 5% retards the 
reaction almost completely. 

These accelerators not merely affect the speed, but also lower 
the temperature range at which appreciable vulcanization takes 

•Probably every rubber chemist has some such formula on which he bases 
changes in curing, and while such figures are only approximate, they do give 
some idea of the order of magnitude of the change in the velocity of the reac- 
tion. The point is of particular commercial importance, because it shows the 
necessity for maintaining cures of rubber articles at exactly the prescribed time 
and temperature. For example, in a cure of 60 minutes at 140C, an error of 
1C would be equivalent to adding from 8 to 10 minutes to the regular cure. 
Sufficient attention has not been paid to this important question, and the only 
reason that more trouble has not resulted is that the maximum in most com- 
pounds is not a point, but extends over a range of some minutes, and this 
automatically provides a certain tolerance. With rapid curing compounds, how- 
ever, the maximum is usually just a point in the curve and a variation in the 
temperature results either in an under, or overcure, 



60 THE ANALYSIS OF RUBBER 

place; some of them, as has been pointed out by Ostromuislenskii, 
Bruni, Bedford and others, increase the speed at ordinary tem- 
peratures to the point where it becomes noticeable. 

One more noticeable action with these organic accelerators is 
the difference in the change in the velocity of the reaction, at 
ordinary temperatures, when the amount of the accelerator is 
varied. For example, 0.05% of the dimethyldithiocarbamate may 
be mixed with rubber, sulfur, and zinc oxide, at a temperature 
around 100C, for some time, without any noticeable effect on the 
rubber. With 0.25% of the same accelerator, in a few minutes a 
decided change takes place, and the rubber becomes hard and 
unworkable, and is clearly partially vulcanized. 

Cold Vulcanization. 

The acid cure process of cold vulcanization consists in sub- 
mitting rubber to the action of sulfur monochloride, either in 
vapor form or in solution. The reaction is similar to that of hot 
vulcanization; the sulfur chloride adding at the double bond, 
forming an addition compound, but in this case, both sulfur and 
chlorine are added, and the resulting compound is different in 
chemical composition, although greatly resembling the hot vul- 
canization product in many of the tensile properties. One im- 
portant fact stands out, that these properties are not as lasting 
in the acid cured as they are in the hot vulcanized rubber. 

The reaction between rubber and sulfur chloride is practically 
instantaneous; consequently, an article to be manufactured by 
this method must first be brought to its final form prior to vulcan- 
ization. It has often been said that the reaction is a surface 
one, but this does not exactly explain the true state of affairs. 
In the case of a sheet of rubber exposed to the vapors of sulfur 
chloride, the gas will be absorbed by the outer surface, but before 
it can diffuse into the center of the sheet, chemical combination 
between the two takes place. This will continue until the surface 
has taken up all of the sulfur chloride with which it can combine. 
In the meantime, especially if the sheet is very thick, the center 
of the sheet is unchanged. A somewhat better distribution of the 
sulfur chloride is effected by swelling the sheet in solvents like 
benzene and then dipping the article in a solution of the sulfur 



THE THEORY OF VULCANIZATION 61 

chloride in benzene. In this way, the penetration of the sulfur 
chloride is facilitated, and better results obtained. 

There is no excess of sulfur chloride remaining as long as the 
rubber is at all unsaturated, and since there is no free sulfur, 
acid cured articles do not show the sulfur blooming so common 
with hot vulcanized articles. 

Vulcanization With Mixed Gases. 

A new method of cold vulcanization through the interaction 
of two gases, has been proposed by Peachey. 4 It consists simply 
in treating a rubber compound with sulfur dioxide, followed by 
hydrogen sulfide. Sulfur is liberated in such an active form that 
it can immediately combine with the rubber. In order to avoid 
the possibility of having sulfuric acid remain in the rubber, it has 
been found advantageous to use the sulfur dioxide first and 
follow this by an excess of hydrogen sulfide, since the latter is 
inert, and will, in time, be lost by diffusion. A control of the 
extent of the vulcanization is obtained by adding exact quantities 
of sulfur dioxide; since an excess of hydrogen sulfide is used, the 
exact amount of sulfur to be added to the rubber can be cal- 
culated. • 

Since this process of vulcanization takes place at ordinary tem- 
peratures, there is no doubt that, if practicable, it can be used with 
many substances as fillers which it is not possible to use under 
present conditions. This is especially true of some of the bright 
organic colors. It is very noticeable, for example, that a much 
wider range and more brilliant colors may be used with sulfur 
chloride vulcanization than with the hot vulcanization. It is, 
however, a question of time and temperature of heating; with the 
ultra-rapid accelerators, it is quite possible that this advantage 
will not be as marked as it is with the much slower accelerators. 

1 British patent 136,716, Feb. 21, 1921 ; cf. also Caoutchouc and Guttapercha 
18, 10744-5 (1921) ; Dubosc claims that in a discussion of the theory of vul- 
canization, he stated that the reaction may be caused by the production of 
colloidal sulfur. He showed that sulfur dioxide and hydrogen sulfide could be 
generated by the ingredients of a rubber compound, and further stated that if 
hydrogen sulfide and sulfur dioxide were simultaneously present, they would 
combine to liberate sulfur in such a form as to enable it to immediately combine 
with the rubber. In this instance, Dubosc was discussing the reaction in con- 
nection with the theory of hot vulcanization, but the latter was merely used as 
a source of the gases mentioned, and not necessarily the temperature at which 
the gases would unite to give off sulfur as indicated. Whether or not this may 
be called an anticipation of Peachey's patents remains to be decided. 



62 THE ANALYSIS OF RUBBER 

Ostromuislenskii's Theories of Vulcanization. 

Much has been said on the subject of the theories of vulcaniza- 
tion advanced by Ostromuislenskii, but if we can maintain our 
definition of vulcanization given in the beginning of this chapter, 
we cannot see that there exists any fundamental difference be- 
tween his theories, and the present-day practice. He has shown 
that at ordinary temperatures, he can cause rubber and sulfur 
to unite in the presence of piperidyl-piperidine-dithiocarbamate. 
With a sufficient amount of accelerator, the same thing can be 
done with dimethyldithiocarbamate, but if we reduce the quantity 
of the accelerator to the neighborhood of 0.05%, then we will 
find that the reaction will be so slow at ordinary temperatures as 
to be commercially negligible. It now becomes merely a question 
of concentration of accelerator in order to make the velocity of 
the reaction at ordinary temperatures, which is so slow as to 
approach zero, approach a finite quantity that will be visible to 
the eye in a reasonably short time. 

A much more distinctive process is the production of a vul- 
canized rubber substance by the addition of trinitrobenzene ; 
with benzoyl peroxide, with halides and halide esters. 5 These 
products have some of the properties of rubber-sulfur vulcani- 
zates. However, it must be concluded that we have here nothing 
to invalidate our present conception of vulcanization, and that 
what has been accomplished is to show that the change from the 
sticky, plastic rubber, which was first thought to be a function of 
sulfur, and was later extended to include sulfur chloride, is really 
a property of a number of substances. Some of these may require 
heating, and some do not; some require the presence of metallic 
oxides, and still others do not. As far as can be seen, the chief 
difference which has been noted up to this time, is the stability 
of the various products vulcanized in the different ways, and it 
may be that in order to arrive at a satisfactory definition of 
what we mean by vulcanization, we shall not only have to state 
that the vulcanized articles shall have certain definite properties, 

' There is an excellent analogy here between the various combinations of 
rubber with elements, or groups of elements, and the similar reactions of the 
unsaturated fatty acids, such as linoleic, linolenic, etc. With sulfur and sulfur 
chloride, we have products quite similar to the addition product with oxygen, 
having many properties in common, such as solubility, etc. 



THE THEORY OF VULCANIZATION 63 

but that the rate of decomposition, or deterioration, shall be not 
greater than a certain set figure. 

To summarize the situation from the analyst's point of view: 
vulcanization is the chemical combination of rubber with other 
substances, without reference to time, temperature, catalysts 
(except as these remain as constituent parts of the mixture), or to 
any of the steps through which the products may have passed in 
reaching the final form in which the rubber is found. For 
example, there should be no chemical difference between rubber 
and sulfur which has combined as such and which has combined 
by reason of the treatment by Peachey's process. 



Chapter "V. 
Sampling. 

The sampling of rubber, and the materials to be used in the 
manufacture of rubber compounds, as is the case with a great 
many other commercial and natural materials, is usually done in 
the most casual fashion, whereas the proper sampling, and the 
care of the sample until the analysis has been completed, is funda- 
mental. Unless the proper precautions are taken to make the 
sample represent the material from which it was taken, and 
maintain its condition and purity, not only is the accuracy of the 
analysis affected, but the incorrect results may frequently lead to 
' false conclusions as to the manufacture or composition of the 
article. Samples for analysis have been packed, without 
adequate protection, in the same package with cans of oil; 
ground rubber samples in unsealed paper envelopes with bits of 
excelsior distributed throughout; inner tube samples which have 
been light checked ; rubber articles with unmistakable evidence of 
having been placed against steam radiators; these do not appeal 
to the analyst as fertile fields for valuable results. Samples of 
less than 1 gr. may be very flattering to the ingenuity and 
ability of the receiver, but they can hardly be said to be repre- 
sentative of lots of finished goods weighing hundreds, or even 
thousands of pounds. 

The process of sampling may be divided into three stages: (a) 
the taking of the sample; (6) its removal to the laboratory; (c) 
the preparation of the sample for analysis. The purpose of these 
three steps is to have the actual material used in making the 
various determinations of the same chemical composition as the 
lot which it represents. If the sample is to represent a number of 
pieces, the sample should be drawn to represent a fair average 
composition of the lot. More often it is not advisable to take 
more than one piece of a lot, or even a part of that. Under such 
conditions, we cannot speak of average composition, but since the 
supposition is that the entire lot is uniform, and that any one 
piece (or part of it), will truly represent, not the average, but 
the exact composition of all of the rest, in such cases we must 

64 



SAMPLING 65 

select our samples at random. Speaking generally, when we 
sample raw materials we should draw more than one sample, 
since these raw materials are apt to vary throughout the lot, 
and also because raw materials are thoroughly mixed in the proc- 
ess of manufacture, and it is the average composition which is 
of chief interest. With finished materials, the averaging process 
has already taken place, and it is a fair risk to assume that the 
lot is uniform. 1 

A. Taking the Sample. If the material is liquid, it should be 
thoroughly stirred before drawing off the sample. Particular 
attention should be taken to note whether there are two liquid 
layers, or whether there is any suspended matter (such as water 
in gasoline, or foots in vegetable oils). The liquid should be 
bottled at once, and sealed with a stopper which is not attacked 
by the liquid. The bottle should be scrupulously clean, both 
inside and outside, and should be dry. 2 Greases, waxes, and 
resins, are usually packed in small containers ; a few ounces may 
be drawn from each container, or from a certain proportion of 
them. These small samples are united to form a composite 
sample, which is mixed and quartered until a final sample of 
about 100 to 200 gr. is obtained. This should be placed in a 
wide-mouthed bottle, or a can, and sealed. 

Dry pigments are usually received in kegs or sacks. As in the 
case of greases, a small portion is withdrawn from some propor- 
tion of the containers; these are united, mixed and quartered, and 
a final sample of 100 to 200 gr. bottled and sealed. 

The sampling of crude rubber is discussed in connection with 
the testing of crude rubber. 3 

B. Transportation to the Laboratory. The distance between 
the place where the sampling occurs, and that where it is to be 
tested, may be a matter of only a few feet, or it may be hundreds 
of miles. The principles involved are the same, irrespective of the 

1 This is particularly true in rubber goods, so far as actual composition is 
concerned ; but such a sample will not reveal any variation in the vulcanization, 
since the latter process takes place in a relatively small number of units. We 
have met cases of rubber belting, for instance, which is vulcanized a portion 
at a time, where the manufacturer paid particular attention to the first and 
last part of the belt because that was where the samples would be taken. No 
amount of testing is proof against such chicanery. 

- Samples of oil have been received, the container being an ink bottle in which 
a few drops of ink were still to be seen at the bottom of the bottle ; and this 
sample was to be tested for mineral acids! 

a Cf. page 22. 



G6 THE ANALYSIS OF RUBBER 

distance. The containers in which the raw materials are sent 
should be tightly stoppered, so as to avoid the introduction of 
dirt and other foreign matter, and also to prevent change in 
composition through evaporation or leakage. 4 Manufactured 
articles sent for analysis should be carefully wrapped. 

The principal deteriorating agents of vulcanized rubber are 
heat, light, and oils. It is quite essential to see that each package 
is not only carefully wrapped, but that it will not come in contact 
with oils, and on packages which are to be sent any distance 
specific instructions should be written on the outside, that such 
packages are to be kept in a clean, cool, dark and dry place. 
These same precautions should be observed in the laboratory, 
after the samples have been received. 

If considerable stress has been laid on the care requisite for 
delivering the sample to the laboratory, our justification is that 
the analyst can test only what he receives; he cannot tell how 
great a change, or even at times that any change at all, has taken 
place. Questionable samples should be discarded at once ; failure 
to do so will often lead to disagreeable controversies, which ac- 
complish no good purpose, and tend to diminish that respect 
which the analyses of the laboratory should inspire. 

C. Preparation of the Samples for Analysis. 

Raw Materials. Pigments, oils, waxes, etc., should be mixed 
thoroughly before each portion is taken for analysis. 

Unvulcanized Rubber Compounds. Sheet out rapidly on a cool 
mill, roll between holland and place in a covered can. 

Reclaimed Rubber. Treat as under unvulcanized rubber com- 
pounds. 

Cements. Cements should be stirred thoroughly before por- 
tions are taken for analysis, and then immediately covered. A 
fair sized quantity should be taken, the solvent removed, prefer- 
ably by evaporation in thin layers at room temperature (if neces- 
sary, to remove the last traces of the solvent, the rubber may be 
heated for a short time between 80 and 90C, but it is better to 
avoid heating of any kind), and the residue sheeted out and 
rolled between holland, as in the case of other unvulcanized 
(•(impounds. 

' A sample of gasoline was sent to the laboratory in a can without a cover. 
It was delivered after working hours, and was not discovered until the next 
morning. Considerable evaporation had taken place, and the residue was hardly 
what it was expected to be. 



SAMPLING 67 

Vulcanized Rubber Samples. Strip off all fabric, and see that the 
rubber is homogeneous, i.e., that there are not two or more com- 
pounds in the sample. 

Grind about 50 gr. in a meat grinder, or coffee grinder, or 
by passing between the tightly closed rolls of a laboratory mill. 
Sift the ground material through a 20 mesh screen until about 
25 gr. has been collected. It is not necessary to sift the entire 
amount of 50 gr. 

The type of grinder is immaterial, providing the following pre- 
cautions are observed: The sample must be ground at room tem- 
perature, without being appreciably heated up; no metal must 
be introduced into the sample during the grinding; and prefer- 
ence should be given to those grinders which tear the sample 
rather than just cut it up, since the former gives the greater sur- 
face for extraction. 

Material containing fabric and rubber in such a manner as to 
make it impossible to produce good separation, shall be cut with 
scissors into as small pieces as is practicable. Rubber and fabric 
cannot be ground together, since segregation will be certain to 
occur on account of the difference in behavior on grinding, and 
this holds true even if the entire sample is ground and sifted. 

Hard rubber samples are prepared for analysis by rasping. 

Insulated wire should be cleaned with a damp cloth, to remove 
any adhering cotton or other adhering material, but care must be 
exerted to see that waxy hydrocarbons are not removed from the 
surface. If, however, a saturated braid sample must be used, 
remove the braid, and sandpaper the insulation for a depth of at 
least .005 in., and wipe with a damp cloth. This treatment will 
probably give low results for waxy hydrocarbons, and hence 
should be resorted to only when absolutely necessary, and a 
statement regarding the treatment given the sample should al- 
ways be included as a part of the report of the analysis. On the 
other hand, it should always be indicated when analyses are made 
on samples which have been braided, or which have been vul- 
canized in contact with a saturated braid or tape, since there will 
be a migration of the liquid hydrocarbons of the saturation from 
the braid or tape, into the rubber insulation, and although the 
waxy hydrocarbons may be a bit low, because of the sandpaper- 
ing of the surface, the acetone extract and the liquid hydrocar- 
bons will be high. 



Chapter VI. 
Extractions. 

Certain organic substances, mainly the oils and waxes, are 
removed by extract with acetone, chloroform, or by saponification 
with alcoholic potash. The results obtained by these three opera- 
tions are largely qualitative, and from them may be obtained a 
fair index as to the quality of the article as a whole. In addition, 
there are some substances which may be determined quite accu- 
rately in these extracts. 

Extraction Apparatus. The extraction apparatus should com- 
ply with the following requirements: It should be of the reflux 
type, with the condenser placed immediately above the cup which 
holds the sample; the sample must be suspended in the vapor of 
the boiling solvent; the cup must be of the syphon type; the cup 
must be far enough away from the sides of the extraction flask 
that it will be maintained at the temperature of the boiling point 
of the solvent; only glass or metal joints may be used — there 
shall be no cork, rubber, or similar material in the extractor, with 
which the solvent may come in contact, and from which extract- 
able matter may be obtained. 

The extraction flask may be of a size that will permit it to be 
weighed directly, or it is permissible to transfer the extract to a 
smaller flask for evaporation, drying, and weighing. The Cottle 
(better known as the Underwriters), the Joint Rubber Insula- 
tion Committee, and Bureau of Standards types are all satisfac- 
tory, and may be relied upon to give equally accurate results, but 
any extractor which fulfills the above requirements will do. 

Acetone Extract. 

The acetone used in this extraction must be redistilled, and 
free from water or acid. It should be distilled over sodium or 
potassium carbonate, and kept in clean dark-colored glass bottles. 

Place 2.000 gr. of the sample in an acetone extracted paper 

68 



EXTRACTIONS 69 

thimble, or fold it in an extracted filter paper; insert in the 
syphon cup, and extract continuously for eight hours. 1 The heat- 
ing must be controlled so that the solvent syphons about 20 times 
per hour. Remove the solvent, dry the flask and contents at 90C 
to constant weight, 2 and calculate to percentage. This figure is 
usually called the "acetone extract, uncorrected." Due record 
should be made of the color and odor of the extract, and of any 
other peculiarities which may be noticeable. With high free 
sulfur, or waxy hydrocarbons, these substances will separate out 
on the sides of the flask. 

Reserve the residue for the chloroform extraction. 

The acetone dissolves the unchanged or free sulfur, vegetable 
fats or oils, rosin, mineral oils, paraffin, ceresin, ozokerite, a con- 
siderable portion of bituminous substances such as the mineral 
rubbers, tars, etc., and the so-called resins of the crude rubber. 
In simple mixtures, the separate constituents may be determined 

1 Eight hours should suffice for auy properly prepared sample extracted under 
exact conditions. However, some uncured samples may fuse together into a solid 
mass, and require a longer time for comparatively complete extraction. In such 
cases, extract until the solution in the extraction cup is colorless, and continue 
for four hours longer. Uncured samples should be sheeted thin and rolled 
between hardened filter paper, to effect a thorough and more rapid extraction. 
The expression "complete extraction" is a misnomer : the free sulfur actually 
is extracted in the first four hours, but the soluble organic matter is extracted 
with difficulty. Additional quantities of extract can be dissolved up to 48 hours, 
or even more, but the amount so obtained is but a small proportion of the total, 
and is more or less constant. Hence, if we interrupt the extraction at a definite 
point, we secure results which serve the purpose of indicating the quality of 
the rubber, and are comparable with other extracts made in a similar manner. 
The same is true to a large extent with the chloroform and alcoholic potash 
extractions, and we really deal with comparable, rather than with absolute 
values. 

The necessity for continuous extraction is explained on the same basis ; with 
samples of approximately the same degree of fineness, the extraction is a matter 
of time, rather than the number of times the syphon empties; hence, standing 
overnight would permit the solvent to extract a considerable quantity of soluble 
matter that would not otherwise be extracted. Many of the variations in check 
results are really due to faulty manipulation, rather than to the type of extrac- 
tor, or fineness of the sample. 

2 There has been considerable discussion as to the adoption of a standard 
time for drying. Some samples are dry in half an hour and it is a waste of 
time to continue for hours longer. On the other hand, the Joint Rubber Insula- 
tion Committee found some samples, notably those high in solid hydrocarbons, 
which were not dry in two hours. Sometimes in the hot, humid months of 
summer, water may condense on the outside of the condenser of the extractor, 
and some of this may find its way into the extraction flask. If it does, it must 
be removed, even if it does take more than half an hour ; it is not extract, and 
must not be weighed as such. Of course, the longer periods for drying may 
lose a little more of the free sulfur than the shorter periods ; especial care 
must be taken to see that the temperature does not go over 90C, for even at 
this temperature, there is some loss of free sulfur and at higher temperatures, 
over an extended period, the loss may be very great. 



70 THE ANALYSIS OF RUBBER 

with some accuracy, but in the more complex ones only a few 
of the constituents may be determined with sufficient accuracy to 
be of any value. The free sulfur may always be determined with 
a high degree of accuracy; in the absence of tars and mineral 
rubber, paraffin and ceresin are capable of being determined with 
equal accuracy. Fatty oils will be associated with the rubber 
resins, and if we assume that the latter are about 3.5 to 4% of 
the rubber present, we may get a fair line on the quantity of 
vegetable oils, but such a scheme is only approximate. 

Rosin may be determined by the method of E. J. Parry. 3 
The fatty acids are dissolved in 20 cc. of 95% alcohol, a drop of 
phenolphthalein is added, and then strong caustic soda (one part 
of alkali to two parts of water) until the reaction is just alkaline. 
The solution is heated for a few minutes, allowed to cool, and 
then transferred to a 100 cc. stoppered graduated cylinder. The 
latter is filled to the mark with ether, 2 gr. of powdered silver 
nitrate is added, and the mixture shaken vigorously for fifteen 
minutes, in order to convert the acids into their silver salts. 
When the insoluble salts have settled, 50 cc. of the clear solution 
(containing the silver salts of rosin) is pipetted off into a second 
100 cc. cylinder, and shaken with 20 cc. dilute hydrochloric acid 
(1 acid to 2 water). The ethereal layer is drawn off, and the 
aqueous layer is shaken twice with ether. The ether extracts are 
united, washed with water, and the ether distilled off in a 
weighed beaker. The residue, rosin, is dried at 110 to 115C, 
cooled, and weighed. 

This is an excellent means of separating fatty oils and rosin; 
it is best performed by taking the water solution in the deter- 
mination of unsaponifiable matter, making it acid with hydro- 
chloric acid, and extracting the liberated fatty acids with ether. 
The ether must be driven off, and the fatty acids dried, before 
the method may be used. 

The mineral oils can be partly separated from hard paraffin, 
sufficiently so as to give some indication of the composition. So 
far as our experience goes, no method has been given which will 
determine the relative amounts of mineral rubber and paraffin 
in a mixture of the two. The possibilities of such a method 
being developed are very remote, in view of the wide variations in 

8 Allen's Commercial Organic Analysis, 4th ed., Vol. V, p. 73. 



EXTRACTIONS 71 

the composition of the mineral rubbers, and the fact that chemi- 
cally they are so nearly like paraffin. 

Chloroform Extract. 

The chloroform extraction is performed in the same apparatus 
used in making the acetone extraction. The chloroform should 
be redistilled over alkali. 

Extract for four hours, the residue from the acetone extraction 
(it is not necessary to remove the acetone adhering to the 
sample), using about 60 cc. of the solvent. If at the end of four 
hours, the solvent in the syphon cup is still colored, continue to 
extract until it is colorless. Filter the extract through fat free 
filter paper into a small Erlenmeyer flask, distil off the solvent, 
and dry the flask and contents to constant weight at 95C. 

If the chloroform extraction cannot be started immediately 
after the acetone extraction has been completed, the sample 
should be protected against oxidation by keeping it in a vacuum 
desiccator in a vacuum of at least 50 mm of mercury. Vulcanized 
rubber which has been extracted with acetone oxidizes very 
rapidly in the air and the resultant products are so soluble in 
chloroform as to yield hopelessly false results, being as much as 
five to ten times the true amount. 

Reserve the residue from the chloroform extraction for treat- 
ment with alcoholic potash. 

The chloroform dissolves part of the rubber, particularly the 
undercured, and the oxidized rubber. Its chief value is that it 
dissolves part of the mineral rubber, the solution taking on an 
intense brown or black color. It is an invaluable qualitative test 
for mineral rubbers, the color being quite distinctive, and not 
likely to be mistaken for anything else. 

The chloroform extract in a well cured and unoxidized sample 
of soft vulcanized rubber, will run from 1 to 3% of the rubber 
present, with the average nearer the lower figure. It has been 
suggested as means of determining whether or not the rubber has 
been undercured, but the data available are largely limited to 
insulation compounds, and are not entirely convincing. 

Alcoholic Potash Extract. 

Dry the residue from the chloroform extraction at 60C until 
the odor of chloroform is no longer noticeable. Place the rubber 



72 THE ANALYSIS OF RUBBER 

in a 200 cc. Erlenmeyer flask, and cover with 50 cc. normal alco- 
holic potash. 4 Boil for four hours under reflux condenser. Filter 
by decantation through a hardened filter paper, wash with two 
portions of 25 cc. of hot alcohol, and then thoroughly with hot 
water. Evaporate the filtrate to dryness, take up in warm water 
and when the solution has been effected, cool to room tempera- 
ture. Transfer to a separatory funnel, add 30 cc. N/5 hydro- 
chloric acid and sufficient water to bring the total up to about 
100 cc. Add 40 cc. of ethyl ether, shake thoroughly, allow to 
stand until the two layers are completely separated, draw off the 
water into a second separatory funnel, and continue to extract 
with 20 cc. portions of ether until a colorless solution results, and 
then twice more. Unite all the ether fractions in the first separa- 
tory funnel, and wash with water until the water shows no 
further acidity (test with silver nitrate solution). Filter the 
ether through a plug of extracted cotton into a weighed beaker or 
flask, evaporate to dryness, and dry to constant weight at 95C. 
Another method for the determination of the alcoholic potash 
extract is to dry the residue from the chloroform extract, cool, 
and weigh. Place the rubber residue in a 200 cc. Erlenmeyer 
flask, add 50 cc. N/1 alcoholic potash, and boil under a reflux 
condenser for four hours. Filter off the rubber on a Gooch or 
alundum crucible, wash with hot alcohol, and then hot water, un- 
til the washings are free from alkali; dry in an inert atmosphere 
to constant weight; the loss in weight is the oil substitute. 5 

* We should not be led astray by those who wish to replace potassium 
hydroxide with sodium hydroxide. When the former was difficult to obtain, one 
did what could be done with the material which was available, but no question 
of a slightly higher cost should interfere now with the use of a better and 
more widely known reagent. On the other hand, with pure grain alcohol diffi- 
cult to obtain under present conditions in the United States, the use of 
methylated alcohol becomes almost obligatory. It is hard to see just what error 
would be introduced by the presence of methyl alcohol ; it is difficult to con- 
ceive of anything which might be present in a rubber compound which is soluble 
in methyl alcohol, and insoluble in acetone, chloroform or ethyl alcohol. If 
the analyst will see that his denatured alcohol has been denatured with methyl 
alcohol, and will use this denatured alcohol only after redistillation over caustic 
potash, the chances for error are very small indeed. Of course, in any event, 
and regardless of the kind of alcohol used, a blank is always run, and due cor- 
rection made for the results found. Again, no careful analyst will use an alco- 
holic potash solution which has been standing a long time, and particularly if 
it is badly discolored. 

6 This method is not as accurate as the previously mentioned one, and is not 
to be recommended. There is the greatest difficulty in washing out all of the 
alkali, and the latter cannot be removed with acids on account of the proba- 
bility of these acids attacking some of the pigments in the rubber. 



EXTRACTIONS 73 

Ordinary crude rubber shows a small amount of material solu- 
ble in alcoholic potash, usually around 1% of the amount of 
rubber. This will be included in the results in either method. 
In the first method, we weigh the fatty acids, although they were 
present originally as the glycerides; i.e., we weigh only 95% of 
the substitute. These two elements tend to neutralize each other, 
and the result is a pretty accurate determination of the amount of 
fatty substitute, not including, of course, any unchanged oil which 
would have been extracted in acetone, or any pigments contained 
in the substitute. 

If vegetable oils are used, and there is sufficient sulfur present, 
we may find that a part of the oil has been converted into an 
insoluble form, and will appear at this point. There is no way 
to distinguish substitute formed during vulcanization and that 
added as such, excepting that the oil in the substitute is usually 
very well changed into the acetone-insoluble form, whereas the 
oil added as such will be changed to only a slight extent. 

The method involving loss of weight is practically worthless, 
because it is an almost hopeless task to thoroughly wash out the 
alkali. In one case continuous washing for 8 hours did not suf- 
fice, and acid cannot be used to neutralize the alkali, on account 
of its effect on the acid-soluble fillers. 

Analysis of Acetone Extract. 

Unsaponifiable Matter. 

Add to the acetone extract, 50 cc. of alcoholic potash, boil under 
a reflux condenser for two hours, and evaporate to dryness. Add 
10 cc. of water and 20 cc. of ether, heat until solution is complete ; 
cool, and transfer to a separatory funnel, wash out with warm 
water, and cool, then with two 20 cc. portions of ether; the separa- 
tory funnel should contain 100 cc. of water, and not less than 40 
cc. of ether. Shake vigorously, allow the two layers to separate, 
and draw off the aqueous layer into a second separatory funnel. 
Repeat the extraction until no further material can be extracted 
(not less than four extractions should be made) . Unite the ether 
portions of the extract, and wash with water until free from alkali 
(the first two portions may be united with the original aqueous 
solution, and the whole reserved for the determination of free 
sulfur) . Filter the ethereal layer through extracted cotton, wash- 



74 THE ANALYSIS OF RUBBER 

ing with ether and hot chloroform, using the latter to rinse the 
original flask, and both separatory funnels. Evaporate to dry- 
ness, dry to constant weight at 95 to 100C, cool and weigh. 

The above method gives the liquid and solid hydrocarbons, and 
the unsaponifiable resins. The difference between the total ex- 
tract, and the sum of the free sulfur and unsaponifiable matter, 
will consist of the saponifiable resins, and any fatty oils which 
may have been extracted. The acetone soluble matter of the 
mineral rubber will be found largely in the unsaponifiable por- 
tion. Rosin, as its composition indicates, will be distributed be- 
tween the two, about 90% being saponifiable. 

Waxy Hydrocarbons. 

The time-honored method for separating the solid paraffins has 
been to dissolve the unsaponifiable portion in alcohol, and freeze 
out the paraffin. However, some of the latter will always remain 
in the alcohol, along with any liquid mineral oils, and the un- 
saponifiable rubber resins. The Joint Rubber Insulation Com- 
mittee devised a method for correcting for the alcohol soluble 
paraffin, in the absence of mineral oils, or, if the latter were 
present, to get the total soluble paraffins and the mineral oil 
together. The alcohol insoluble paraffins are called "Waxy hydro- 
carbons A" and the soluble paraffins are called "Waxy hydrocar- 
bons B." If the latter are solid, the sum of the two is the total 
paraffin in the sample. 

If it is desired to know only the total mineral hydrocarbons, 
then the method for Waxy hydrocarbons B is used directly. 

Waxy Hydrocarbons A. 

Add 50 cc. absolute alcohol to the unsaponifiable matter and 
warm until solution is as complete as possible. Cool the solution 
to — 4 or — 5C, and maintain at this temperature, or lower, by 
packing the flask in a mixture of ice and salt. Filter out the 
waxy hydrocarbons, using a funnel packed with ice and salt and 
applying suction if necessary. Wash the flask and filter with 
25 cc. of 95% alcohol which has been previously cooled to the 
same temperature. Dissolve the residue on the filter paper in 
hot chloroform into the original flask; evaporate the chloroform, 
and dry the residue at 95 to 100C to constant weight. 



EXTRACTIONS 75 

Waxy Hydrocarbons B. 

Evaporate the alcohol from the determination of Waxy hydro- 
carbons A, add 25 cc. of carbon tetrachloride, and transfer to a 
separatory funnel. Shake with cone, sulfuric acid, drain off the 
discolored acid, and repeat with fresh portions of the acid until 
there is no longer any discoloration. Vigorous shaking is abso- 
lutely necessary for the success of the method. After drawing 
off all of the acid, wash the carbon tetrachloride solution with 
repeated portions of water until all traces of acid are removed. 6 
Transfer the carbon tetrachloride solution to a weighed flask, 
evaporate off the solvent, and dry to constant weight at 95 to 
100C. Note whether the residue is solid, liquid, or pasty. 

• On account of the specific gravity of the carbon tetrachloride washing with 
water is a very tedious proposition, because the carbon tetrachloride must be 
drawn off with each washing, and returned to the flask. While the Joint Rubber 
Insulation Committee did not recommend it, the carbon tetrachloride may be 
diluted with ether until the mixed solvents have a gravity lower than that of 
water ; the washing can then be continued as usual with separatory funnel 
washings. Ether to the extent of about two and a half to three times the 
volume of carbon tetrachloride will be necessary to have the ether-tetrachloride 
mixture float on the water layer. 



Chapter VII. 
The Determination of Rubber. 

It is a peculiar fact concerning the analysis of rubber that the 
determination of the principal constituent involved is seldom, if 
ever, made by a direct determination. A tremendous amount of 
research has been undertaken, methods and revisions of methods 
have been suggested, but as yet no one method has succeeded in 
securing the endorsement of the rubber analysts. 

The methods for the determination of rubber may be classified 
under three headings: (1) direct; (2) indirect; (3) difference. 
In No. 1, the idea is to form a definite compound with rubber 
and either weigh the compound directly or determine some part 
of the compound and from these figures to calculate the total 
rubber. The two principal methods in this group are the tetra- 
bromide methods and the nitrosite methods. The indirect 
methods (No. 2), proceed to separate the rubber, but the latter 
is not determined as such, but is determined as the loss during 
the solution. The difference methods comprise the third group, 
and the principle involved is merely to determine every other 
known constituent, subtract the total from 100%, and call the 
remainder rubber. 

The Tetrabromide Method. 

The tetrabromide method was first advocated by Budde, for 
use in determining the rubber in unvulcanized compounds, or 
crude rubber. The bromination solution used was 6 gr. of bromide 
and 1 gr. of iodine in 1000 cc. of carbon tetrachloride. The rubber 
was swollen in carbon tetrachloride, and filtered. The clear solu- 
tion was treated with 50 cc. of the bromine solution, allowed to 
stand for 24 hours, diluted with an equal volume of alcohol, and 
when the precipitate had settled it was filtered and washed with 
carbon tetrachloride-alcohol (1-1), and finally, to remove the bro- 
mine, with alcohol. The precipitate was weighed as C 10 H 16 Br 4 , 
and calculated to rubber, using the factor 0.298. 

76 



THE DETERMINATION OF RUBBER 77 

The gravimetric method did not prove successful, and evoked 
considerable criticism. Fendler, Harries, Hubener, Spence, and 
others, presented various modifications, and, in the meanwhile, 
Budde had published a volumetric method, which he claimed was 
satisfactory for rubber vulcanized with sulfur chloride. In this 
method, the rubber swollen in carbon tetrachloride was treated 
with the brominating mixture, and after the tetrabromide had 
been filtered free from bromine, it was treated with cone, nitric 
acid and N/5 silver nitrate, and the bromine determined as in 
Volhard's method. 

The volumetric method did not prove any more acceptable than 
the gravimetric. For some compounds, good results were ob- 
tained, but in others, especially with vulcanized rubber, it was 
found that the bromine did not replace the sulfur of vulcaniza- 
tion. There was also found to be a loss of bromine during the 
acid treatment, which Spence corrected by fusing the tetra- 
bromide with alkali. Vulcanized samples gave low results, but 
when it was noted that the sulfur combined with the double bonds 
of rubber in stoichiometric proportions, it was seen that by adding 
to the bromine found in the tetrabromide the bromine equivalent 
of the sulfur of vulcanization (2Br = S) , the results were more 
uniform, and more nearly correct. It was also noticed that 
hydrobromic acid was formed during bromination, and efforts 
were made to eliminate this factor by freezing the brominating 
solution, but, on the whole, while cooling reduced the formation of 
hydrobromic acid, it did not eliminate it. 

Recently, further attempts have been made to make the method 
practical. Lewis and McAdam 1 published a modification based 
on Mcllhenny's 2 method for the determination of substitution, 
and Fisher, Gray and Merling 3 have recommended some im- 
provements in the Lewis and McAdam method. 

When bromine adds to rubber, whether the latter be vulcanized 
or not, there are a number of ways in which the reaction may 
progress: 

(1) HC : CH + 2 Br = HCBr.HCBr 

(2) HCBr.HCBr = HC : CBr + HBr 

(3) HC : CBr + 2 Br = HCBr.CBr 2 

(4) HC : CH + HBr = HCBr.CH 2 " 

(5) CH 2 .CH 2 + Br 2 = CHBr.CH 2 + HBr 

1 J. Ind. Eng. Chem. 12, 675-6 (1920). 
3 J. Am. Chem. Soc. U, 1084 (1899). 
3 J. Ind. Eng. Chem. is, 1031-4 (1921). 



78 THE ANALYSIS OF RUBBER 

The first reaction is purely additive; the second is a splitting 
off of HBr, re-forming the double bond, which again combines 
with 2 Br as shown in No. 3; the liberated hydrobromic acid may 
unite at a new double bond, as in No. 4; and finally, No. 5 is a 
case of straight substitution. The resulting product will contain 
HCBr.HCBr; HCBr.CH 2 ; HCBr.CBr 2 ; HBr and Br. It is ap- 
parent that every molecule of HBr remaining uncombined with 
the rubber represents a loss of 2 Br from the excess over that 
required for the double bonds. This has been one of the serious 
errors, and it is one which varies greatly with variations in the 
condition of time, temperature, and concentration of the solutions. 

In determining the iodine number of "burnt" linseed oils, Smith 
and Tuttle 4 found that concordant results could be obtained only 
when a very exact procedure was followed, in which the weight 
of the sample, volume and strength of the iodine solution, time 
and temperature of the reaction, were specified within very nar- 
row tolerances. The analogy in chemical reactions, between dry- 
ing oils and rubber, is very striking, and we may expect to find 
just as great difficulties with vulcanized rubber as with oxidized 
linseed oil. Lewis and McAdam brominate for 2-4 hours, while 
Fisher, Gray and Merling say 2.5 to 3.5 hours. The amount of 
the sample is quite indefinite, not over 2.00 gr. for unvulcanized; 
1.50 to 2.00 gr. for vulcanized. In the latter case, no special at- 
tention seems to have been paid to whether the material contained 
30 or 90% of rubber hydrocarbons; nor to whether they are deal- 
ing with rubber having a vulcanization coefficient of 2.0 or 10.0. 
There is a considerable amount of lack of definiteness in a reac- 
tion which, in a similar determination (adding halogen to partly 
oxidized oils), has been shown to be absolutely essential. It is 
perhaps not to be wondered at that after so many years of trial, 
there still remains such an element of doubt. 

In spite of the fact that it requires an extra determination 
(sulfur of vulcanization), the tetrabromide method offers an 
excellent opportunity for a direct method, if some one will take 
the time to ascertain the exact conditions which are necessary 
for consistent results. 

*J, In<J, and Eng. Cheni. 6, 994 (1914). 



THE DETERMINATION OF RUBBER 79 

Nitrosite Method. 

The nitrosite method was first described by Harries. 5 He 
found that by allowing N 2 3 to react for a sufficient time with 
a solution of rubber, he obtained a final product of constant 
composition, C 10 H 15 N 3 O 7 , and by weighing this material, the rub- 
ber hydrocarbon could be calculated. Rubber resins were re- 
moved by extraction with acetone. Alexander in attempting to 
repeat Harries work, claimed that during the formation of the 
nitrosite, carbon dioxide was given off, and that the composition 
of the final nitrosite of Harries was really C 9 H 12 N 2 O c . The loss 
of carbon dioxide has not been confirmed by any other workers in 
this field, so that if carbon dioxide was really lost, as Alexander 
says, it was because of some condition not in accordance with 
Harries method. However that may be, every one agreed upon 
the difficulty in getting a constant composition for their end 
product, and this method was more or less abandoned until Wes- 
son 6 took it up from a new angle. He proved that Alexander 
was wrong in his statement that carbon dioxide was lost, and 
hence, although the final product varied in composition, it still 
held all of its original carbon. If, therefore, the carbon in the 
nitrosite was determined, a simple calculation could be made 
directly to rubber which did not depend upon any definite com- 
position of the nitrosite and was independent of the sulfur of 
vulcanization. Wesson secured some good results on a few com- 
pounds, but with some complicated commercial compounds, large 
differences cropped out. Tuttle and Yurow, 7 while investigating 
the possibilities of Wesson's method, found that his best results 
were obtained by a fortunate balancing of errors, and that when 
the causes for these errors were removed, accurate results could 
be obtained directly in the presence of practically any known 
organic or inorganic fillers. The only unfortunate circumstance 
connected with this method is the fact that it requires a fairly 
complicated combustion train, and this has prevented many 
laboratories from testing it out. Perhaps further experimentation 
along the lines of wet combustion would simplify it sufficiently to 
permit its more general adoption. In the meantime, it stands as 

°Ber. 81,, 2991-2 (1901) ; 35, 3256: 4429 (1902) ; 36, 1937 (1903). 
• J. Ind. EDg. Chem. 5, 398 (1913) ; 6, 459-62 (1914) ; 9, 139-40 (1917). 
7 India Rubber World, 57, 17-8 (1917) ; Bureau of Standards Technologic Paper 
145 (1919), 



80 THE ANALYSIS OF RUBBER 

our only accurate direct method for the determination of rubber, 
irrespective of the condition of the rubber product, and the degree 
of vulcanization. The method is as follows: 

The preliminary extractions with acetone, chloroform, and 
alcoholic potash will show whether mineral rubbers or oil sub- 
stitutes are present. If the former, then acetone and chloroform 
extractions are necessary ; and with oil substitutes, make acetone 
and alcoholic potash extractions; and if both are present, make 
all three extractions. When an alcoholic potash extraction is 
made, wash the sample thoroughly with 5% hydrochloric acid, 
hot water and alcohol. 

Take 0.500 gr. of the finely ground sample (call this weight 
W) , and extract with acetone for eight hours (make other extrac- 
tions, if necessary, as stated above) . Dry the residue in hydrogen 
(or other inert gas) for two hours at 100C. Place the sample in 
50 to 75 cc. of chloroform and allow it to swell. Pass into this, 
until the green color which is formed persists for 30 minutes, the 
gases formed by heating arsenic trioxide and nitric acid of 
specific gravity 1.30. To avoid contamination, it is important 
that no rubber connections be used. Immerse the flask contain- 
ing the rubber in cold water during the nitration. Allow the solu- 
tion to stand overnight; the next day, filter off the nitrosite 
through a Gooch crucible and wash with small quantities of 
chloroform. Remove the acid gases and chloroform from the 
flask by means of a gentle current of air. Evaporate the filtrate 
to dryness. Dissolve the nitrosite remaining in the flask in the 
Gooch crucible and in the residue from the filtrate in acetone, 
and filter the solution through asbestos into a weight-burette. 
The total volume should be about 100 cc. Allow this solution to 
stand for a short time to permit any sediment which may form to 
settle out in the bottom of the weight burette. 8 Weigh the 
burette before and after filling, calling the difference N. Draw 
off about 25 cc. into a small Erlenmeyer flask, reweigh the 
burette, and call the difference O. Evaporate the portion drawn 
off to a small volume, transfer to a porcelain boat (about 14 cm. 
long and 1 cm. wide), which has been filled with alundum, and 
wash the flask with acetone (it is best to make this transfer in 
small portions, drying the boat and contents for a few minutes be- 

8 The usual type of weight burette will not answer ; it is necessary to have 
the solution drawn off at the side, about an inch from the bottom. See B. of 
S. Tech. Paper 145 for a sketch of the weight burette. 



THE DETERMINATION OF RUBBER 81 

tween each addition). After the final washing and drying, add 1 
or 2 cc. of 1% solution of ammonia in distilled water, 9 and dry in 
an inert gas for one hour at 90C. Repeat with a second portion of 
ammonia and dry as before. By this means, all of the organic 
solvent will be removed. 

Place the boat in the furnace, and proceed with the combus- 
tion. Pass the products of combustion through U-tubes or other 
satisfactory absorption tubes, placed in the following order: a, 
b, c, potassium bichromate-cone, sulfuric acid; d, 20-mesh 
powdered zinc; e, f, soda-lime and calcium chloride; g, potassium 
bichromate-cone, sulfuric acid; h, dilute palladium chloride solu- 
tion, (very little palladium chloride is needed; use about a drop 
of a 10% solution in 10 cc. of water) . Weigh e, f , and g before 
and after each combustion; refill c and g frequently from the 
same stock solution, so that the gases which enter e, and those 
that leave g, will have the same moisture content. The pal- 
ladium chloride serves to detect the presence of carbon monoxide 
or other reducing gases; if there is any blackening, it shows in- 
complete oxidation, and, in this event, the results must be dis- 
carded and the determination repeated. 

The carbon dioxide will equal the algebraic sum of the differ- 
ences in tubes e, f, and g. Call this sum P. The factor for cal- 
culating from carbon dioxide to rubber hydrocarbons, is .309. 
The formula is therefore as follows: 

n w — — = % of rubber hydrocarbons. 

u x vv 

Correct this figure for whatever extractions were made previ- 
ous to nitration. 10 

The Indirect Methods. 

The indirect methods comprise those in which the rubber is 
decomposed and rendered soluble in some solvent, and the ma- 
terial which goes into solution is called rubber hydrocarbons. A 
great many solvents have been suggested, heavy petroleum (B.P. 
230-280) , anisole, phenetole, xylol, paraffin oil, camphor oil, tere- 

• The nitrosite precipitate is insoluble in distilled water, and in acids ; it is 
soluble in aqueous alkaline solutions. Ammonium hydroxide is used simply be- 
cause it provides the necessary alkalinity, is volatile, and, even if it is not 
completely driven off, introduces no error whatsoever. 

10 In case the extractions contain other material than rubber, the correction 
applied must be an arbitrary one. It will be recalled that the true resin con- 
tent of high grade rubber is about 4% or less, and the normal chloroform and 
alcoholic potash extracts of rubber are about 2% and 1% respectively. 



82 THE ANALYSIS OF RUBBER 

bene, turpentine, salol, nitrobenzene, aniline, cumene and cymene. 
The general procedure is the same for all of them; the acetone, 
chloroform and alcoholic potash extractions remove the soluble 
organic fillers; the solvent is then added and heated until the 
rubber passes into solution. The fillers are separated by filtration 
and weighed, and the loss is vulcanized rubber. Sometimes 
special steps are taken to bring the fillers into a state in which 
they can be easily filtered. All sorts of solvents are used to 
wash the fillers. 

The principal objection to these methods is that one can never 
be sure that there are no organic or inorganic fillers passing into 
solution, or that insoluble organic compounds are not formed 
from the rubber and solvent. Correction after correction must be 
applied, until finally, instead of a determination of rubber, we 
have almost a complete system of analysis. 

Difference Methods. 

There is really no sharp line dividing the difference methods 
from the indirect methods, but we have reserved the term "differ- 
ence" methods for those which remove the rubber by ignition. 
Such methods are widely used, largely because they are rapid, 
and it is a fact that they are frequently quite accurate, espe- 
cially in the absence of organic fillers. In most cases, the sample 
is subjected to acetone, chloroform and alcoholic potash extrac- 
tions, and then ignited. At times, only the acetone extraction is 
made. A more complicated, and more exact procedure, is that 
of the Joint Rubber Insulation Committee. 11 This method is 
intended for 30% and 40% insulation compounds, but may be 
used for any compound containing no organic fillers. Mineral 
rubber, lampblack, glue, cellulose, etc., all give high results. The 
method is as follows: 

Add to the rubber residue from the alcoholic potash extrac- 
tion, sufficient water to make the total volume 125 cc, and then 
add 25 cc. cone, hydrochloric acid. Heat for one hour, decant 
through a Buchener funnel, using hardened paper, and wash with 
25 cc. of hot water; repeat this treat twice. The rubber should 
be white, and free from black specks from undissolved fillers 
(lead sulfide). Wash the rubber free from chlorides, transfer the 
rubber to the filter paper, dry as much as possible by suction, 

11 Am. Inst. Elec. Eng., April, X917. 



THE DETERMINATION OF RUBBER 83 

wash with 50 cc. of 95% alcohol, and transfer the entire residue 
to a weighing bottle. Dry to constant weight at 95 to 100C. Let 
this weight be represented by C. 

On a portion D of this residue C, determine the ash E (see 
below) and the sulfur F in ash E. Determine the sulfur H in 
another portion G of residue C. 

To determine E, place about 0.50 of residue C into a weighed 
porcelain crucible, heat gradually until the crucible has ceased 
to smoke, then raise the temperature and heat until all organic 
matter is destroyed. Cool and weigh, calling the residue E. If E 
is small, the determination of sulfur in the ash may be omitted, 
and F assumed to be zero. From the data thus obtained, the 
percentage of rubber hydrocarbons in the original material is 
calculated as follows: 

— 1 1 — — ^ -p- ) = % rubber hydrocarbons. 

The simplest method of all, is the determination of the ash. 
Wrap a 1 gr. sample in filter paper, and extract with acetone for 
four hours; ignite the residue in a porcelain crucible; cool and 
weigh. Correct for the sulfur in the ash by adding a few drops 
of nitric acid-bromine mixture, heat on the steambath, then add 
5 gr. sodium carbonate, dry carefully until all moisture is 
removed, fuse the residue, extract with hot water and filter; make 
the filtrate just acid with hydrochloric acid, heat to boiling, add 
barium chloride solution, and determine the barium sulfate as 
usual. Deduct the sulfur thus found from the residue on ignition, 
and the difference is called "ash, sulfur free." 

In the ash method, rubber is the difference between 100% and 
the sum of the total sulfur, ash (sulfur free), and any other 
determinations of fillers which may have been made. 

The only good word which can be said for the ash determina- 
tion as a correct method for the determination of rubber, is that 
it requires only a crucible, a Bunsen burner, and a balance. 12 

12 It is quite within reason that those analysts who have for years been 
adhering to the "ash method," will find fault with this statement. With a few 
simple compounds, satisfactory results may be obtained, but beyond this nothing 
is certain. Our experience with this method, in testing rubber materials for the 
U. S. Government over a number of years, demonstrated only too clearly how 
easy it was to be led astray by results obtained with the ash method. The 
errors in this ash method are frequently very large — it is only the occasional 
determination which comes with 2 or 3% of the truth. The plain truth of the 
matter is that the ash method is used because it involves little labor, and re- 
quires but a short time to complete, but if a reasonable degree of accuracy is not 
necessary, one is tempted to ask "Why make the determination at all?" 



Chapter VIII. 

Sulfur Determinations. 
Total Sulfur. 

Sulfur may occur in rubber compounds in any of the follow- 
ing forms: 

(a) Free sulfur. 

(b) Sulfur combined with the rubber. 

(c) Sulfur in organic compounds (mineral rubber, oil sub- 
stitutes, accelerators). 

(d) Sulfides (zinc, antimony, mercury, lead, cadmium). 

(e) Sulfites and sulfates (calcium, barium, lead). 

In addition to the above, there are other substances, such as 
barium carbonate, lead oxide, etc., which, while not containing 
sulfur, are important factors in deciding what method is satis- 
factory for the determination of sulfur. We may say that all 
rubber compounds will contain classes (a) and (6), but beyond 
that, nothing definite may be assumed. It is obvious that if many 
of these sulfur-bearing substances are present, the determination 
of total sulfur becomes a difficult proposition, and, moreover, the 
results obtained decrease in value. 

In looking over the foregoing list, it is obvious that three steps 
are essential: (1) oxidation of organic and inorganic substances; 
(2) fusion of inorganic substances; (3) separation of metals 
forming insoluble sulphates by filtration of the alkaline solution 
of the fusion. 

The purpose of the oxidation is obvious; the oxidizing agent 
must be sufficiently effective to oxidize rather large amounts of 
free sulfur (frequently 5%, and possibly 10% or 12%). Since it 
is assumed that oxidation will be accompanied or followed by 
fusion, it is not essential that the oxidizing treatment be carried 
to the point of the complete oxidation of all of the organic sub- 
stances. The period of fusion must suffice to convert insoluble 

84 



SULFUR DETERMINATIONS 85 

into soluble sulfates. These steps are fairly well agreed upon, 
although various means are suggested for this part of the proce- 
dure. The third step, nitration, is still a subject of controversy, 
although why it should be so is difficult to understand. If the 
solution is acidified before filtration, we may expect to form 
calcium, lead, and barium sulfates. Calcium sulfate is highly 
soluble, lead appreciably so, and barium sulfate scarcely at all. 
Calcium may have been present originally as the carbonate 
(whiting) , or as the sulphate (in antimony compounds) ; the lead 
as oxide, sulfide, sulfite, or sulfate, and barium as the carbonate 
or sulfate. The purpose of the filtration from an acid solution is 
to thus eliminate the insoluble sulfates originally present in the 
rubber mixture, but this will be accomplished only when barium 
sulfate is the only sulfate originally present, and lead and calcium 
are present in very small amounts, and barium carbonate is 
absent. It is evident that only on rare occasions will the condi- 
tions be such as to permit the nitration of the solution of the 
melt from an acid solution, and with compounds of unknown 
composition, it is impossible. 

In the earliest attempts to determine the sulfur in rubber, 
Henriques oxidized with cone, nitric acid. Later, Alexander 
suggested sodium peroxide; Hinrichsen, a modification of Gas- 
parini's electrolytic oxidation (afterwards improved by Spence) ; 
Waters and Tuttle employed cone, nitric acid and bromine; 
Pontio, manganese peroxide ; Frank and Markwald, fuming nitric 
acid; and Kaye and Sharpe fused directly with zinc oxide and 
potassium nitrate. Some advocate a solution, others direct fusion, 
while practically all of those who had a preliminary wet oxida- 
tion added a fusion later. Solution without fusion is obviously a 
faulty procedure in the presence of lead and barium salts, and no 
data have as yet been presented to show that the direct fusion 
methods are accurate when the free sulfur is high. Of the 
methods employing both solution and fusion, that of Waters and 
Tuttle has given the most consistent results with all types of 
compounds, especially when some minor changes from that 
originally proposed are employed. The method recommended is 
as follows: 

Place 0.500 gr. of rubber in a porcelain crucible of about 50 cc. 
capacity, add 20 cc. of cone, nitric acid saturated with bromine, 
cover the crucible with a watch glass, and allow it to stand for 



86 THE ANALYSIS OF RUBBER 

one hour. Heat the crucible gently for one hour, then remove 
the watch glass, rinsing it with little water, and evaporate the 
solution to dryness (with pure gum compounds before evaporat- 
ing add 0.1 to 0.2 gr. of potassium nitrate). Add 5 gr. of 1-1 
mixture of sodium carbonate and potassium nitrate, and 1 or 2 
cc. of distilled water; digest for a few minutes, and then spread 
the paste along the sides of the crucible, and dry on a steambath. 
Fuse the mixture, being careful to avoid contamination of sulfur 
from the flame. When the fusion is cold, place the crucible and 
contents in a beaker with about 250 cc. of water and heat for 
several hours; filter off the insoluble carbonates, washing with hot 
water. The total volume of the filtrate should be between 300 
and 40 cc. Add 7 to 8 cc. cone, hydrochloric acid, cover the 
beaker, and heat on the steambath. Add 10 cc. 10% barium 
chloride, and allow to stand overnight; filter off the precipitated 
barium sulfate, ignite carefully over a Bunsen flame, cool and 
weigh. Calculate to sulfur, using the factor 0.1373. 

The principal change from the published method is the addi- 
tion of the potassium nitrate before evaporating off the nitric 
acid; it is necessary only in the absence of any basic fillers, and 
serves the purpose of changing any free sulfuric acid into potas- 
sium sulfate. With sulfuric acid, there is some danger of it being 
reduced by the organic matter, and sulfur lost as S0 2 . 

Probably the weakest feature of this method is the wear and 
tear on the crucibles, if this indeed can be considered a weak 
point. There are some makes of crucibles which will not last 
through a determination ; on the other hand, some American and 
Japanese crucibles last through five to ten fusions. At this rate, 
the cost is negligible. It might be added further, in speaking of 
crucibles, that the smaller and thinner crucibles last longer than 
the thicker ones ; much of the crucible trouble has been due to the 
use of extra large and thick crucibles, and to the use of inferior 
makes. 

Occasionally, one finds the statement that the solution of the 
fusion should be evaporated to dryness with hydrochloric acid 
to get rid of the last traces of nitric acid, and to render insoluble 
the silica in the alkali silicates which have been formed. The 
small amounts of nitrates remaining after the fusion will not 
appreciably affect the accuracy of the determination and 



SULFUR DETERMINATIONS 87 

dehydration of the silica is unnecessary, as Hiliebrand x has 
shown. However, it is essential that the alkaline solution should 
be reasonably cool when the acid is added, that undue excess of 
acid be avoided, and that the solution be not allowed to concen- 
trate to any large extent. We have frequently found consider- 
able amounts of silica when the solution containing the precipi- 
tated barium sulfate had been allowed to concentrate to 50 cc. 
or less. 

Allen and Johnston 2 have shown that the precipitate of barium 
sulfate formed in the presence of alkali chlorides and hydrochloric 
acid, is contaminated with chlorine and alkalies, and have 
worked out a method for correcting these errors, and so arrive 
at the true sulfur value. The precipitate obtained in the deter- 
mination of total sulfur in rubber is subject to these same errors, 
but if the barium chloride be added rapidly to the hot solution, 
the solution never heated to boiling, and, further, if it is allowed to 
stand for at least 18 hours before filtering, the contamina- 
tion will be low, and the fortunate balancing of errors will give 
results very close to the truth, so much so that it will not ordina- 
rily pay to take the time for the corrections suggested by Allen 
and Johnston. It should be noted, however, that any attempt to 
improve the method of precipitation by eliminating only one of 
the errors, will yield results which are not as accurate as if the 
method was not changed. 

If the free sulfur is low, the fusion method of the Joint Rubber 
Insulation Committee 3 will be found acceptable: 

Mix 0.500 gr. of rubber with 4 gr. sodium peroxide and 6 gr. 
potassium carbonate in a dry 15 cc. iron crucible, and cover. 
Insert the crucible in a hole in a heavy brass plate so that about 
two thirds of the crucible projects through the hole. Heat cau- 
tiously until the first part of the reaction has taken place, and 
then increase the heat until the mixture fuses. Remove the 
flame and cool; place the crucible and cover in a porcelain cas- 
serole containing 200 cc. of water, add 5 to 10 cc. of bromine 
water, and boil until the melt is dissolved. Allow the precipitate 

1 Analysis of silicate and carbonate rocks, U. S. Geological Survey Bull. 422, 
p. 198. 

2 J. Am. Chem. Soc. 32, 588-617 (1910) ; see also Richards and Parker, Proc. 
Am. Acad. SI, 67 (1896) ; Hulett and Duschak, Z. Anorg. Chem. 1,0, 196 (1904) ; 
John Johnston and L. H. Adams, J. Am. Chem. Soc. S3, 829-45 (1911). 

3 J. Ind. Eng. Chem. 6, 75-82 (1914). 



88 THE ANALYSIS OF RUBBER 

to settle, decant the solution through a thick filter and wash with 
hot water. 4 Make the filtrate faintly acid with hydrochloride 
acid, heat to boiling, add 10 cc. of 10% barium chloride solution, 
allow to stand overnight; filter the barium sulfate as usual. 

This method was originally recommended for testing insulated 
wire, in which the free sulfur was limited to 0.7%, and was found 
quite satisfactory. Tuttle and Isaacs 5 found that with high free 
sulfur, the results obtained were not accurate. It has been sug- 
gested that by increasing the quantities of sodium peroxide and 
potassium carbonate, even these high free sulfur samples could 
be analyzed without any trouble, but data are lacking in support 
of this contention. 

It would be extremely desirable, from the time and labor-sav- 
ing points of view, if the oxidation of the free sulfur and the 
fusion could be accomplished in one treatment. Spence's 6 elec- 
trolytic method eliminates the fusion, in the absence of lead and 
barium salts. Evans and Merling 7 have devised a method, 
using a Parr calorimeter: 0.200 gr. of rubber is packed in sodium 
peroxide, with some sugar and potassium chlorate. The ignited 
mass is extracted with water, filtered, acidified, and the sulfur 
precipitated as usual. The authors claim to have secured some 
excellent results so far, and the time required is very little, but 
it seems desirable that others test this method to discover its 
limitations, and faults, if it has any. 

Free Sulfur. 

The procedure to be adopted for determining the sulfur in the 
acetone extract depends largely upon the nature of the material, 
and whether it is desired to make further separation of the 
constituents in the extract. If not, the simplest, and yet the most 
accurate method we have, is as follows: 

To the dried extract, add 100 cc. of water, and 3 to 5 cc. of 
bromine. (If a very high free sulfur is indicated by the character 
of the extract, the amount of bromine should be increased.) 
Allow the flask to stand for half an hour to an hour, boil off 
the bromine, and when the solution is practically colorless, filter 

• The original method called for dehydration of silica, but, as previously noted, 
this is unnecessary. 

6 J. Ind. Eng. Chem. 7, 658 (1913). 
•J. Ind. Eng. Chem. k, 413 (1912). 
1 India Rubber World, 64, 658 (1921). 



SULFUR DETERMINATIONS 89 

through a folded filter into a small beaker; cover the beaker, 
heat to boiling, add 10 cc. of 10% barium chloride, and after 
standing overnight, determine the barium sulfate as usual. 

This method determines all of the sulfur in the extract; a great 
many checks have been run by taking the insoluble residue, fus- 
ing it with sodium carbonate and potassium nitrate, and deter- 
mining the sulfur as is done in the Waters and Tuttle method, but 
the sulfur has never exceeded 0.01 to 0.02% in this residue. The 
oxidation is complete, rapid, requires no evaporation, furnishes 
its own acidity by the reaction with the barium chloride ; in fact, 
after ten years, there still remains to be found some objection to 
its use. 

If it is desired to make further examination of the acetone 
extract, the method of the Joint Rubber Insulation Committee 8 
is recommended: the method starts where the acetone extraction 
has been treated with alcoholic potash, the alcohol removed, the 
residue taken up in water, extracted with ether, and the ether 
washed with water. 

To the aqueous solution, add 2 gr. potassium nitrate; evaporate 
to dryness in a nickel or silver dish, and heat to quiet fusion. 
Transfer to a beaker, neutralize with hydrochloric acid, add 2 cc. 
of acid in excess, filter and wash, making the filtrate up to 200 cc. 
Heat to boiling, add a slight excess of barium chloride solution, 
allow to stand overnight, and determine the barium sulfate as 
usual. 

Kelly 9 calls attention to the fact that what we have been deter- 
mining as free sulfur is not the true free sulfur, but includes, in 
addition to the sulfur which may be said to be available for 
further vulcanization, such amounts of sulfur which may have 
been combined with the organic resins extracted by acetone. 
Obviously, this is so, and in the data presented by Kelly, which, 
however, covers only one compound, there is about 0.40% of 
sulfur combined with the organic matter in the extract. In such 
cases, the free sulfur as determined in the past is quite misleading. 
It is still a very great question as to whether the sulfur will 
always be of the same order of magnitude as Kelly indicates. It 
would have been very helpful if commercial samples had been 

*Loc. cit. 

•The determination of the true free sulfur, and the true sulfur of vulcaniza- 
tion; J. Ind. Eng. Chem. 12, 875-8 (1920). 



90 THE ANALYSIS OF RUBBER 

treated — in the many samples we have tested, we have frequently 
found samples where the free sulfur, as determined by the bro- 
mine method, was less than 0.10%. Upton 10 gives figures for 
free sulfur on some samples of insulated wire, determined by two 
methods, and several of these were below 0.20%, and in one case 
only 0.07%. Without questioning the force of Kelly's argument, 
it does seem as though we needed more data to show what varia- 
tions there are between the free sulfur as at present determined, 
and the amount he calls the true free sulfur. 

Kelly's method is as follows: 

The acetone extraction is performed as usual. The acetone is 
driven off at not over 65C. To the residue, add 50 cc. of 75% 
alcohol which has been saturated with sulfur. Weigh the flask 
and contents to 0.5 gr. Heat for a few minutes, to get the organic 
matter in solution, and then cool slowly. Allow to stand three 
hours; reweigh, and add 75% alcohol, sulfur- free, to replace the 
loss. Decant the solution, wash two or three times with 75% 
alcohol saturated with sulfur, and then dry. The sulfur may 
then be determined by any satisfactory method. 

No word is said as to what is to be done with the alcoholic 
solution of the resins; no scheme has been given for weighing 
them, and at 65C the extract would not be sufficiently dried to 
take that figure as the total acetone extract. If one knew just 
how much alcohol was used, and the sulfur it contained, the 
solution could be evaporated to dryness in a weighed flask, dried 
to constant weight at 90C, and the organic extract determined 
after making due allowance for the sulfur in the alcohol. As it 
stands now, the new method requires a separate extraction for 
the acetone extract. 

Caspari X1 gives a similar method to the one used by Kelly. 
He uses petroleum spirit, boiling point 60-100C, which dissolves 
3.0 gr. of sulfur per litre, whereas the 75% alcohol dissolves only 
0.08 gr. Kelly says nothing about the accuracy when mineral 
rubber, tars, or paraffins are present, whereas Caspari speaks of 
the difficulty in getting these in solution. However, as we are 
interested only in separating the true free sulfur from the sulfur- 
carrying organic substances, it is quite within reason that undis- 
solved paraffin would create no impassable difficulties. 

>"J. Ind. Eng. Chem. 10, 518 (1918). 

11 India Rubber Laboratory practice, p. 110. 



SULFUR DETERMINATIONS 91 

Sulfur of Vulcanization. 

It is often desirable to know the amount of sulfur actually 
combined with the rubber during the process of vulcanization, 
both as regards determining the extent to which it has proceeded 
and to attain a greater uniformity in manufacturing practice. 
The simplest method for estimating uniformity, for comparative 
results, is by means of stress-strain curves, but mechanical de- 
fects operate to change values, so that comparisons are at best 
difficult and uncertain. The noticeable effect on the vulcanization 
by slight changes in sulfur content, demonstrate that the amount 
of sulfur which actually unites with the rubber is the controlling 
feature of the vulcanization. The value for the sulfur of vulcani- 
zation is necessary for the calculation of the total rubber hydro- 
carbons in some of the direct methods, and a further use is the 
possible discovery of the presence of reclaimed rubber in a rubber 
compound. 12 

Several possibilities are available, depending upon the nature 
of the rubber compound. The simplest case is that of pure rub- 
ber and sulfur, and occurs but seldom in commercial articles, 
although it is overworked as a formula for determining the value 
or properties of crude rubber. In this case, if the total sulfur is 
S, the free sulfur Sf, the percentage of rubber 100' — ■ S then the 

sulfur coefficient, Sv will be: — 

~ S — Sf 

100 — s 

In samples containing no organic sulfur compounds, the fol- 
lowing method, based upon the determination of sublimed white 
lead by Schaeffer, 13 gives excellent results: 

The sample is extracted with acetone for eight hours, and the 
free sulfur determined in the extract by the bromine method. 
The residue is placed in a porcelain boat, and transferred to a 

,2 This is not as simple a proposition as it was before the rapid accelerators 
came into use. With inorganic accelerators, the proper cure for rubber was 
approximately at a coefficient of 3.0 to 3.5. and hence higher coefficients were 
fair indications of the presence of reclaimed rubber, especially in connection 
with other qualitative tests. Today, the value of the coefficient of vulcanization 
is almost nil, when, by the use of appropriate accelerators, good cures can be 
obtained with sulfur coefficients below 2.0. Of course, if one can learn what 
accelerator has been used, and determine the coefficient for the best cures with 
that accelerator, such data might be quite valuable in determining the condition 
of the rubber in the sample under observation, 

13 J. Ind. Eng. Chem. J,, 837 (1912). 



92 THE ANALYSIS OF RUBBER 

hard glass tube. Carbon dioxide is passed through the tube, 
which is then heated, gradually at first, and then at a dull red 
heat for a few minutes. The organic matter, together with the 
rubber, is distilled out, but the mineral sulfides and sulfates are 
unchanged. The sulfur in the fillers is determined by transferring 
the residue to a porcelain crucible, and determining the sulfur 
therein by the Waters and Tuttle method for total sulfur. The 
calculations for this method require a separate determination of 
rubber, R. Calling the sulfur in the residue Sr, then the sulfur 
coefficient will be calculated as follows: 

« _ S — (Sr + Sr) 
R 
This is the same formula as before, when R = 100 — S, and 
Sr = 0. 

The most difficult case is when, in addition to sulfides and sul- 
fates, we have organic substances containing sulfur, such as 
oil substitutes, mineral rubber, etc. There are several procedures 
which may be followed, but the safest is probably to use Wes- 
son's nitrosite method as revised by Tuttle and Yurow. 14 In his 
original article, Wesson says: "If the statement of Alexander 15 
proves to be true that the sulfur of vulcanization of the rubber 
remains quantitatively in the nitrosite, this method could pos- 
sibly admit of the simultaneous determination of the sulfur of 
vulcanization. An aliquot portion of the clear acetone solution 
of the nitrosite would be evaporated to dryness, and the sulfur 
determined in the usual way." A few attempts were made to 
determine the sulfur of vulcanization in this way, but not until 
after the errors which were contained in Wesson's method had 
been eliminated, was it possible to secure accurate determination 
of the rubber, and until then, little effort was made to determine 
the sulfur of vulcanization. When the final revision was in shape, 
determinations of the sulfur of vulcanization were found to check 
very well. 

The sulfur coefficient figured by this method, is the result ob- 
tained by dividing the combined sulfur by the percentage of 
rubber hydrocarbons; such a calculation leaves no opening for 

14 As a matter of fact, this method can be used for any compound, and is not 
confined in its application to this single case where organic sulfur compounds 
are present ; it is equally effective in rubber sulfur mixtures, and with mixtures 
containing mineral sulfur bearing fillers. 

15 Z. Apgew, Chem, 20, 1364 (1907) ; 2h 687 (1911) ; Ber. 1,0, 1077 (1907). 



SULFUR DETERMINATIONS 93 

questions as to whether or not the sulfur was combined with 
the rubber, or with something other than rubber. It is simple, 
direct, and accurate. 

When possible to make it, the direct determination of the 
sulfur coefficient (or for that matter any determination) is pref- 
erable to the difference methods, since all questions regarding 
interfering substances are eliminated. Kelly points out that 
not only is the figure usually determined as free sulfur really a 
mixture of elemental sulfur and sulfur combined with the resins 
and other soluble constituents of the rubber, but that part of 
the sulfur insoluble in acetone is soluble in alcoholic potash. 
There seems to be no doubt that our use of the term free sulfur 
is not exactly correct; and that some of the residual sulfur 
should be removed by alcoholic potash seems equally reasonable, 
but, for ordinary length cures, the amount so removed is small 
(Kelly shows 0.07% for 2V 2 hours) . 

If we figure our coefficient on only the sulfur that is insoluble 
in alcoholic potash, obviously we should also take into our cal- 
culations the non-rubber constituents, and this would include 
the acetone soluble matter, or resins. In our formula, we would 
therefore have to correct R for the acetone extract A, and the 
alcoholic potash extract P, and we would have to deduct the 
sulfur in the alcoholic potash extract, Sp; hence, we would have 
the rather involved equation: 

S— (Sf + Sr + Sp) 
bV ~~ R— (A + P) 

As a matter of fact, the relative amounts of rubber and non- 
rubber substances insoluble in acetone are such that even making 
this additional correction changes the coefficient very slightly, 
certainly within the limits of experimental error, as far as our 
experience goes. Hence, although no doubt the published data 
for coefficients of vulcanization are not absolute values, they are 
probably relatively accurate, and are comparable. Hence any 
deductions which may have been made from these data are no 
doubt just as valid as though every correction had been made. 

Sulfur in Fillers. 

The sulfur in fillers is determined as given under the method 
for the determination of rubber by the ash method (cf. page 83). 



Chapter IX. 

Detection of Organic Accelerators. 

There is very little published work on this subject; probably 
a few laboratories have some special tests of their own, but as 
yet no one has seriously taken up this field. The data given 
below is largely from Twiss and Martin, 1 and Earle L. Reed. 2 

Paranitrosodimethylaniline. Extract about 10 gr. of the 
sample with acetone, and dry the extract; add 5 cc. dilute hydro- 
chloric acid, shake thoroughly, and filter. A pink or carmine 
color results if p-nitrosodimethylaniline is present. If the filtrate 
is colorless, divide it into two portions, using one to test for 
aniline, and the other for hexamethylenetetramine. 

The above test is a better negative than a positive test — if no 
color develops, the accelerator is not present, but there may be 
other organic bases which will give a pink color on acidification 
with dilute hydrochloric acid. 

Twiss and Martin call attention to the color of the acetone 
extract which, however, is too common a color to use as an indica- 
tion of an organic accelerator. A more positive test is to treat 
the dried acetone extract, or a dilute hydrochloric acid extract 
of a finely ground sample, with hydrogen sulfide water and ferric 
chloride solution, forming a blue, or greenish-blue, if paranitro- 
sodimethylaniline is present. The reaction depends upon the re- 
duction of part of the accelerator during vulcanization, to 
p-aminodimethylaniline, which, when treated as stated, forms 
methyleneblue. 

Twiss gives the following alternative method: treat the hydro- 
chloric acid solution of the dried acetone extract with a small 
piece of metallic zinc. Filter off the solution, cool thoroughly, 
and add a well cooled dilute aqueous solution of sodium nitrate. 
Add a small amount of this mixture to a solution of beta-napthol, 
with excess of aqueous sodium hydroxide. A deep blue results 
in the presence of p-nitrosodimethylaniline. 

'Rubber Age, 9, 379-80 (1921). 
2 Unpublished data. 

94 



DETECTION OF ORGANIC ACCELERATORS 95 

It can also be tested for by means of the Liebermann reaction. 
The dried acetone extract is boiled with a small amount of dilute 
caustic, and filtered; the filtrate is evaporated to dryness, cone, 
sulfuric acid and phenol added, the mixture diluted with water, 
and made alkaline with caustic potash, when a deep blue colora- 
tion will appear. 

Aniline. Using the hydrochloric acid filtrate after testing for 
paranitrosodimethylaniline, add a drop of freshly prepared and 
filtered solution of bleaching powder. A violet color indicates 
the presence of aniline. Thiocarbanilide will ordinarily give no 
reaction to this test, unless present in very large amounts. It is 
well, in order to make sure of its absence, to take another portion 
of the dried extract, and heat, and look for the characteristic 
odor of thiocarbanilide. 

Thiocarbanilide. A portion of the dried acetone extra ct is 
placed in a test tube, stoppered, and connected by a delivery 
tube with a second test tube containing two or three cc. of distilled 
water. The delivery tube must dip below the surface of the 
water. The first test tube is now heated until bubbles escape 
through the water in the second test tube, after which the heat- 
ing is continued strongly for two or three minutes. Test the 
water in the second test tube for aniline with the filtered bleach- 
ing powder solution; a violet color will indicate thiocarbanilide 
if the original aniline test was negative. 

Thiocarbanilide has a very characteristic odor, which is 
especially noticeable when heated. Heat the dried acetone ex- 
tract, and compare the odor with that of some heated thio in a 
second test tube. 

Hexamethylenetetr amine. Using the second portion of the 
hydrochloric filtrate from the test for p-nitrosodimethylaniline, 
add 5 cc. of water, 1 cc. of phosphoric acid, a small amount of 
phenylhydrazine hydrochloride, 2 drops of 10% ferric chloride 
solution, and 2 drops of cone, hydrochloric acid. A cherry red 
color is produced by the formaldehyde from the hexamethylene- 
tetramine. 

Extract a ground sample with water, and test the extract for 
ammonia with Nessler's solution. A positive test indicates alde- 
hyde ammonia or hexa — although some of the less commonly 
used accelerators may yield small amounts of ammonia, and 
hence respond to this test. 



96 THE ANALYSIS OF RUBBER 

Diphenylamine. To the dried acetone extract from about 10 
gr. of finely ground sample, add 2 cc. of cone, sulfuric acid, and 
agitate gently. Add a small crystal of sodium nitrate — a blue 
coloration results if diphenylamine is present. 

This test can be made directly on light-colored compounds by 
placing a few drops of cone, sulfuric acid on the rubber to be 
tested, dipping a glass rod in dilute nitric acid, and touching it 
to the edge of the sulfuric acid. 

Quinoidine. Treat the dried acetone extract with dilute sul- 
furic acid; quinoidine gives a blue fluorescence. Rochelle salts 
precipitate the tartrates of quinine or cinchonidine, but not 
quinidine. A saturated solution of potassium iodide added to an 
acid solution gives quinidine hydroiodide. Quinine and quinidine 
give the thalleioquin test, but cinchonine and cinchonidine do 
not; to a solution of the acetone extract in dilute sulfuric acid, 
add very weak bromine water, drop by drop, until a faint yellow 
persists, but avoid an excess of bromine; add ammonia, drop by 
drop, when a brilliant green color results. Making this solution 
acid turns the color to red. 

General Tests. Extract 10 gr. of finely ground sample with 
dilute hydrochloric acid, cool thoroughly, and diazotize with cold 
dilute aqueous sodium nitrite. (The simplest scheme is to put a 
small piece of ice in the solution during the diazotizing; it can 
be removed later.) After a few minutes, pour a little of this 
mixture into a solution of beta-napthol in excess of caustic soda; 
a red precipitate or coloration indicates the presence of a primary 
aromatic amine, such as aniline toluidine, p-phenylenediamine, 
etc., or of derivatives of such bases with aldehydes (formaniline, 
methyleneaniline, benzylidene-aniline) , and with carbon bisulfide 
(thiocarbanilide, or triphenylguanidine). 



Chapter X. 

Mineral Analysis. 

The first step in a fillers determination of a rubber compound 
is to make a qualitative analysis of the metals which it contains. 
In this work, the color of the sample will be of considerable 
assistance in cutting out unnecessary steps, as will also a knowl- 
edge of the use to which the article is to be put. Only in the 
black compounds is there any necessity for making a fairly com- 
prehensive examination. 

Preparation of the Solution. The possible presence of lead, 
barium and calcium in a mixture containing sulfur (as sulfuric 
acid) makes the problem of making up a solution for qualitative 
analysis quite an interesting one. While several choices are open, 
the following procedure is recommended because of the fact that 
it permits quantitative separations to be made on a number 
of elements: 

Place exactly 2.500 gr. of finely divided rubber in a porcelain 
casserole (about 250 cc. capacity), cover with 25 cc. of fuming 
nitric acid, and after standing in the cold for 15 to 30 minutes, 
covered with a watch glass, heat on a steam bath or hot plate 
until the rubber and all other organic matter is entirely de- 
stroyed. Potassium chlorate and fresh acid should be added from 
time to time. Evaporate the solution to dryness, add hydro- 
chloric acid and a little water, and again evaporate to dryness 
and heat to dehydrate silica. Take up the residue with 50 cc 
boiling water and 2 or 3 cc. of cone, hydrochloric acid. Filter 
into another porcelain casserole, and repeat the evaporation and 
dehydration of silica. Take up with 50 cc. of hot water, and 2 
or 3 cc. of cone, hydrochloric acid as before, and filter. Unite 
the two portions of insoluble matter, and reserve for further 
treatment. 

Heat the filtrate from the above, and add, drop by drop, 10 
cc. of barium chloride solution until no further precipitate is 
formed, and then a few drops in excess. Allow to stand over- 

97 



98 THE ANALYSIS OF RUBBER 

night, filter off the barium sulfate (which may be discarded), 
wash well and transfer the filtrate to a 250 cc. graduated flask. 

The insoluble portions reserved above are fused with sodium 
carbonate in a nickel crucible, cooled, and the melt taken up 
with distilled water. If lead, barium, or calcium sulfates were 
in the insoluble residue, they will now appear as insoluble car- 
bonates, while the silica, if any, will be in solution. Filter off 
the insoluble matter, wash free from alkali, and then dissolve 
the carbonates off the filter with dilute hydrochloric acid and hot 
water. Filter through the same filter paper, and unite the filtrate 
with the solution already in the graduated flask. 

The filtrate from the separation of the carbonates contains 
the silica; it should be evaporated to dryness, and the silica 
dehydrated and determined in the usual way. The filter paper 
from the filtration of the lead and barium should be ignited, and 
examined for silicates which may not have been attacked during 
the fusion. 

The solutions united in the graduated flask are now made up 
to the 250 cc. mark at room temperature; 50 cc. of this solution 
contains the fillers from 0.500 gr. of rubber. 

By this procedure, we have eliminated the sulfuric acid, which 
would prove so troublesome with lead, barium, and calcium, but 
in so doing, have introduced barium into the solution. This is 
of no importance, for barium is usually determined on a separate 
sample by a short but excellent method. 

Another element is introduced through the fusion in a nickel 
crucible, but nickel is not likely to be found in rubber compounds 
so that we need merely eliminate it in its turn, and proceed with 
our analysis. On account of lead, fusion in platinum is impos- 
sible, while fusion in iron would introduce serious complications. 

The object in making up a standard solution, is that 50 cc. may 
be taken for qualitative analysis, and further aliquot portions 
may be drawn for such quantitative tests as may be desired. 
In fact, with so few metals to be determined, 1 it is frequently 
possible to combine qualitative and quantitative separations at 
the same time. 

If the silica is less than 0.5%, we may assume that it came 

1 Lead, iron, aluminium, zinc, calcium, and magnesium are practically the only 
metals to be determined. Antimony and barium are determined in special tests ; 
manganese will be encountered where iron oxides are present, but is not neces- 
sarily determined. 



MINERAL ANALYSIS 99 

from the talc used in dusting, and that the silica pigments, such 
as tripoli, talc, asbestine, aluminum flake, etc., have not been used 
as fillers. 

The procedure for making the qualitative and quantitative 
separations may be taken from the standard text books, and 
need not be repeated here. A few words of caution may not 
come amiss. 

In only two cases has vermilion been found amongst many 
hundreds of samples tested; it is too costly, and since it is used 
only for its color, there should be little difficulty in detecting this 
substance from the color of the compound. 

Green-colored samples should be tested for arsenic, not that it 
is likely to be found, but merely to be on the safe side. Arsenic 
colors should never be used in rubber compounding, but it is well 
to see that no one is taking a chance. 

Copper, even in traces, should be carefully looked for, because 
even in small amounts its deteriorating influence on rubber com- 
pounds is remarkable. 

Note whether or not there is any appreciable quantity of mag- 
nesium; a small amount may be expected from the talc used in 
dusting stocks in the mill room, but it should be only a matter 
of 0.10% or so. More than that requires a quantitative deter- 
mination, owing to the practice of using small amounts of mag- 
nesium oxide to activate organic accelerators. 

If the nitric acid solution of the rubber shows insoluble 
material, and yet no silica is present, it indicates insoluble sul- 
fates of lead or barium, or both. 

Black specks remaining after the fuming nitric acid treat- 
ment of the rubber, indicates gas black or lamp black, for which 
a separate determination is made. 

It will be seen from the description of the mineral fillers used 
in rubber manufacture, that the following metals may be found: 
antimony, lead, iron, aluminium, chromium, zinc, calcium, 
barium, magnesium, sodium, and ammonium salts. The com- 
pounds formed with these metals, consist of oxides, sulfides, 
sulfites, sulfates, carbonates, and silicates. 

Oxides. The oxides are usually determined by difference ; after 
the determination of the acid radicles, the excess of bases over 
that required to combine with the acids is assumed to be present 
as oxide. 



100 THE ANALYSIS OF RUBBER 

Sulfides. Stevens 2 determines the sulfide sulfur as follows: 
The apparatus consists of a Kipp generator for carbon dioxide, 
a 250 cc. flask with an inlet tube reaching nearly to the bottom 
of the flask, and a ground-in stopper carrying an outlet tube 
(an all-glass wash bottle can readily be adapted for the pur- 
pose), and connected to the outlet tube are two absorption 
bottles containing lead acetate solution. Place in the flask 10 
cc. of cone, hydrochloric acid and 20 to 30 cc. of ether, pass a 
current of carbon dioxide through the apparatus until all air is 
removed, then remove the stopper and add the sample (0.1 to 1.0 
gr., depending upon the amount of sulfide expected; where noth- 
ing is known regarding the sample, use 1.0 gr.). Again pass car- 
bon dioxide through the apparatus for about 30 minutes, with an 
occasional shaking of the flask. During this period, the hydro- 
chloric acid attacks the sulfides, liberating hydrogen sulfide, 
which is carried over to, and absorbed by the lead acetate solu- 
tion. The purpose of the ether is to swell the rubber, and facili- 
tate the penetration of the acid to all parts of the sample. 

Heat gently to drive off the ether and the final traces of 
hydrogen sulfide. Reserve the solution in the flask for the deter- 
mination of sulfate sulfur. All of the sulfide sulfur is now com- 
bined with the lead. 

Stevens determines the sulfur from this point by adding acetic 
acid to the lead acetate solution in order to decompose the car- 
bonates formed, the lead sulfide is filtered off, and washed free 
from lead salts, transferred to a stoppered flask, a standard 
iodine solution added, and after the reaction is complete the 
excess of iodine is titrated with sodium thiosulfate. However, 
any other accurate method will answer the purpose; the lead 
sulfide may be dissolved in nitric acid, taken to fuming with 
sulfuric acid, and the lead sulfate determined gravimetrically. 

If pure nitrogen is available for sweeping out the apparatus, 
it will be found to be much simpler to use sodium hydroxide for 
absorbing the hydrogen sulfide; the solution can be oxidized 
with bromine, and after acidification, the sulfate can be precipi- 
tated with barium chloride; altogether, much simpler, and 
probably more accurate than the lead acetate method. 

Sulfide sulfur, excepting antimony sulfides, may also be deter- 
mined by the ignition method of Schaeffer, transferring the resi- 

2 Analyst, 1,0, 275-81 (1915). 



MINERAL ANALYSIS 101 

due to a flask similar to the one recommended by Stevens, and 
proceeding as directed by him for driving over the hydrogen 
sulfide. This procedure is best for lead sulfide; antimony and 
mercury sulfides sublime unchanged. 

Sulfites. Sulfites and sulfates are transposed by heating with 
sodium carbonate. Schaeffer gives the following method for 
determining the sulfite-sulfur in sublimed blue lead: 

Boil 1.5 gr. of the sample with 3 gr. of sodium carbonate; 
allow to stand, filter, and wash thoroughly. To the filtrate, add 
3 cc. of bromine water, heat gently to oxidize the sodium sulfite 
to sulfate, and precipitate the sulfate with barium chloride. The 
barium sulfate formed will contain both the sulfur present as 
sulfite, and sulfate; deduct the amount of sulfur present as 
sulfate from the total, and the remainder is calculated to lead 
sulfite. (See determination of sulfates in the presence of sulfites, 
under sulfate-sulfur.) 

Sulfates. Stevens determines the sulfate-sulfur in the residue 
from the determination of sulfides, as follows: Extract the resi- 
due with hydrochloric acid until no further material can be dis- 
solved; unite the filtrates, and determine the sulfur as usual. 
It will be noted that by this means Stevens dissolves out only 
the lead sulfate and calcium sulfate; barium sulfate will be only 
slightly attacked. This method is therefore not applicable for 
the determination of lithopone, for example, or in any other case 
where barium sulfate is present along with some sulfide. 

We again find Schaeffer's ignition process of value in deter- 
mining the sulfates. Boil the ignited residue with sodium car- 
bonate as directed under sulfite-sulfur, and filter. The function 
of the bromine water in the sulfite determination is to oxidize 
the S0 2 to S0 3 ; if instead of adding bromine water we add 
hydrochloric acid, and boil the solution, the sulfur dioxide will 
be driven off, and we will have remaining only the sulfate-sulfur. 

Carbonates. Carbonates can be determined in an apparatus 
similar to Stevens' arrangement for sulfide-sulfur. Instead of a 
Kipp generator, we use air which has first been passed through 
a soda-lime tower, to remove traces of carbon dioxide. In this 
case, the absorption train consists of two absorption bottles con- 
taining cone, sulfuric acid and potassium bichromate (a and b) ; 
two soda-lime tubes (c and d) ; and the fifth tube containing sul- 
furic acid and bichromate (e). It is vital in this determination 



102 THE ANALYSIS OF RUBBER 



that tubes b and e should be frequently refilled, and from the 
same solution ; only with such precautions are we able to main- 
tain the air at the same moisture content when it leaves e as 
when it entered c. Tubes c, d, and e, are weighed before and 
after the determination; the increase in weight is the carbon 
dioxide. Cases are known where d actually lost weight, owing 
to the fact that c absorbed all of the C0 2 , and the air withdrew 
from d some of its moisture, which, however, was reabsorbed by e. 

Any similar arrangement will do just as well, providing 3 the 
gas used to wash the apparatus contains no carbon dioxide, or 
organic matter which might be oxidized by the sulfuric acid- 
bichromate mixture; the absorption tubes are adequate for the 
purpose; and the balance of the moisture content of the gas is 
preserved. 

Silicates. These have already been separated by the method 
of getting the metals of the fillers into solution. It is only neces- 
sary here to repeat that all of the silica is not obtained by the 
first dehydration and treatment with hydrochloric acid, no matter 
how long the process be continued; the operation must be re- 
peated or the error will show up in the determination of the other 
constituents. 

Special Determinations. 

The qualitative and quantitative analyses made as prescribed 
in the preceding paragraphs will suffice for the determination of 
most of the metallic bases, or fillers, but some of these are better 
determined by special tests; amongst the mineral fillers we find 
in this list the antimony compounds, lead chromate, barium car- 
bonate, etc., and amongst the organic, carbon black, blue, etc. 

Antimony. The principal trouble with antimony is getting it 
into solution without loss. There should be little difficulty once 
this has been accomplished. Rothe 4 treats the sample with 
10-20 cc. cone, nitric acid and 2 cc. sulfuric, and heats for 1 
to 2 hours at a moderate heat; then increase the heat until 
all nitric acid is driven off and the sulfuric acid fumes strongly. 
More nitric acid is added, and taken to fuming, and this opera- 
tion is repeated until the absence of darkening shows that the 

•For a more complete discussion on this point, see Tuttle and Yurow. "The 
Direct Determination of Rubber by the Nitrosite Method," U. S. Bureau of 
Standards Tech. Paper, No. 145 (1919). 

«Chem. Ztg. S3, 679 (1909). 



MINERAL ANALYSIS 103 

organic matter is destroyed. Dilute to 100 cc. and boil to expel 
all nitric fumes. Schmitz 5 takes from 2 to 4 gr. of finely cut 
rubber (Frank and Marckwald think the quantity is too high, as 
it no doubt is for most antimony compounds) , and treats it in a 
Kjeldahl flask with 15 cc. cone, sulfuric acid per gram of rubber. 
One drop of mercury and a small piece of paraffin (to prevent 
foaming) are introduced. Heat until the solution starts to clear; 
add 2-4 gr. of potassium sulfate, and heat until colorless. 
Cool, dilute with water, add 1 to 2 gr. of potassium bisulfite, 
with excess of tartaric acid ; heat until no sulfur dioxide remains, 
add dilute hydrochloric acid, filter, and titrate the antimony. 
Wagner 6 fuses in a porcelain crucible, 0.5 to 1.0 gr. of rubber 
with 5 gr. of 1-4 sodium nitrate-potassium carbonate. The 
rubber is mixed with part of the fusion mixture, placed in the 
crucible, and covered with the remainder. The heat must be 
applied gradually, and if any organic matter remains, more 
sodium nitrate must be added, and the whole reheated. Wagner 
claims good results, but the method looks risky; the danger of 
loss of antimony by excessive heating is very great. When zinc 
oxide or sulfide are present, Frank and Marckwald 7 separate 
the rubber from the fillers with xylol ; otherwise, they oxidize the 
organic matter with cone, nitric acid and potassium chlorate, 
finally evaporating with hydrochloric acid. If organic matter is 
still present, it must be eliminated. The antimony is precipi- 
tated as sulfide, and weighed as such. Collier, Levin and 
Scherrer 8 take advantage of the simultaneous determination of 
the fillers by the cymene method to determine the antimony after 
the rubber has been dissolved out. Their method is as follows: 

Extract 0.500 gr. of the sample with acetone for 8 hours, 
and with chloroform for 4 hours. Dry the residue in a vacuum 
desiccator, transfer to a 300 cc. lipped assay flask, add 25 cc. of 
cymene, and heat on an electric hot plate at 130-140C until the 
rubber is dissolved. Cool the flask, dilute with 250 cc. of petro- 
leum ether, and allow to stand overnight. Filter by decantation 
through a tight Gooch pad of asbestos, previously washed with 
alkali, cone, hydrochloric acid, and water, and dried. Wash by 
decantation with petroleum ether until the filtrate is colorless. 

'Gummi Ztg. 25, 1928-30 (1911). 

•Chem. Ztg. SO, 638 (1906) ; J. Soc. Chem. Ind. 25, 583 (1906). 

T Gummi Ztg. 23, 1046 (1909). 

•Rubber Age, 8, 104-5 (1920). 



104 THE ANALYSIS OF RUBBER 

Add 30 cc, of cone, hydrochloric acid to the assay flask, and 
shake until all of the antimony sulfide has gone into solution; 
filter slowly through the Gooch, using gentle suction. Wash 
thoroughly, and dilute the filtrate to 250 cc. with hot distilled 
water, pass in hydrogen sulfide until the antimony has been com- 
pletely precipitated. 

After the solution of the antimony has been effected, it may 
be determined by any of the well known methods. Wagner, and 
Frank and Marckwald weigh as sulfide, Schmitz recommends 
titration, as do Collier, Levin and Scherrer. The methods recom- 
mended by the last named are as follows: 

Filter off the antimony sulfide, wash with hydrogen sulfide 
water, and transfer the precipitate to the filter paper. Place 20 
cc. of concentrated hydrochloric acid in the beaker, and set aside 
temporarily. Transfer the antimony sulfide and the filter paper 
to a Kjeldahl flask, add 12-15 cc. of concentrated sulfuric acid 
and 5 gr. of potassium sulfate, place a funnel in the neck of the 
flask, and heat until the solution is colorless. Wash the funnel, 
and dilute the solution to about 100 cc. with water, add 1-2 gr. 
of sodium sulfite, transfer the hydrochloric acid in the beaker 
in which the antimony sulfide was precipitated to the Kjeldahl 
flask, and boil until the sulfur dioxide is all driven out. Dilute 
to 250-275 cc. with water, cool to 10-15C, and titrate with per- 
manganate until a faint pink color is obtained. 

Instead of filtering the antimony on filter paper, it may be 
filtered on a Witt plate and asbestos. Transfer the plate, pad 
and precipitate to an Erlenmeyer flask; remove any antimony 
sulfide adhering to the beaker or funnel with hydrochloric acid. 
Wash the beaker and funnel with hot distilled water, dilute the 
solution to 250-275 cc, add 12 cc. of concentrated sulfuric acid, 
boil the solution until no trace of hydrogen sulfide is obtained 
with lead acetate paper, cool to 10-15C, and titrate with stand- 
ard permanganate solution. 

Barium Salts. Ignite a 1 gr. sample in a porcelain crucible, 
cool, add 3 to 5 drops of nitric acid and 1 cc. of water, and stir 
into a paste, add 5 gr. of 1-1 potassium nitrate-sodium carbonate, 
dry on the hot plate or steam bath, fuse until the melt is soft 
or pasty; allow it to cool, extract with hot water, and wash with 
hot water containing a little sodium carbonate. Dissolve the in- 
soluble carbonates in hydrochloric acid, and wash the filter paper 



MINERAL ANALYSIS 105 

thoroughly. Nearly neutralize the nitrate with sodium carbon- 
ate, and pass hydrogen sulfide through the solution until the lead 
is entirely precipitated. Filter, heat the filtrate to boiling, and 
add 10 cc. of 10% sulfuric acid; allow the precipitate to stand 
overnight, and determine the barium sulfate as usual. 

The only troublesome element is lead, and it may be com- 
pletely eliminated. Check determinations of 0.10% of the 
barium sulfate present may easily be obtained. 

In some specifications, a maximum limit is placed on the total 
sulfur, but barytes is a permissible filler, without having the 
sulfur which it contains count as part of the total sulfur. In 
such cases, the determination of barytes is obligatory; if made by 
this method, the error in the total sulfur caused by the correc- 
tion will not exceed 0.02%. 

Barium Carbonate. 9 Place 1 gr. of the sample in a porcelain 
boat, and ignite in an atmosphere of carbon dioxide as described 
by Schaeffer. 10 After ignition, and when the ash is at room tem- 
perature, remove the boat, grind the ash to a fine powder in an 
agate mortar, transfer to a 250 cc. beaker, cover with 5-10 gr. of 
ammonium carbonate, 15-20 cc. of strong ammonia, and 50 cc. of 
distilled water. Ammonium carbonate transposes lead sulfate 
into lead carbonate, but is practically without action on barium 
sulfate. Boil the mixture for 15 to 30 minutes, filter, and wash 
the precipitate thoroughly to remove all soluble sulfates. Wash 
the residue on the filter paper back into the original beaker with 
distilled water, add 10 cc. glacial acetic acid, and sufficient water 
to make the volume up to 100 cc. Heat to boiling, and filter 
through the same filter paper as before. Lead, barium calcium 
and zinc carbonates pass into solution, whereas lead sulfide and 
barium sulfate are not attacked. Pass hydrogen sulfide into the 
filtrate, filter off the lead sulfide, heat the filtrate to boiling, and 

9 The reason for working out a method for determining barium carbonate is 
not without interest. In material made under specifications, some manufac- 
turers evidently desired to use compounds which contained more than the pre- 
scribed amount of sulfur. Realizing that the specifications exempted the sulfur 
in the barytes from counting in the total sulfur, and knowing that the barium 
sulfate was being estimated from the amount of barium found by analysis, they 
felt that by adding barium carbonate, they would receive credit for sulfur 
equal to the barium in the carbonate, and thus bring the total within the 
specification limit. The trick was first discovered when, after correction for the 
sulfur supposed to be present in combination with the barium, the total sulfur 
was actually less than the free sulfur. 

10 Cf. page 91. 



106 THE ANALYSIS OF RUBBER 

precipitate the barium with 10 cc. of 10% sulfuric acid. Allow 
to stand overnight, and determine the barium sulfate as usual. 

If barium sulfate and no carbonate is present, a small amount 
of precipitate will be found, showing a slight solubility of the 
barium sulfate, or else reduction of the sulfate to sulfide. The 
amount will usually be less than 1% of the amount of barium 
sulfate present. In a mixture of the two, the carbonate will run 
somewhat high, for the same reasons, but with proper attention 
to details the results will be quite sufficient for every purpose. 

Gas Black or Lamp Black. Chemical analysis alone will tell 
nothing as to whether gas black or lamp black has been used. 
Even the microscope is, as yet, of little value in distinguishing 
between the two, and the only thing remaining for us to do is to 
determine the total carbon, and assume from the physical prop- 
erties of the article, whether or not the black is gas black or 
lamp black. 

The free carbon is determined as follows: X1 

Extract 0.5 gr. of rubber for 8 hours with a mixture by volume 
of 68% chloroform and 32% acetone. Transfer the sample to a 
250 cc. beaker, and heat until it no longer smells of chloroform. 
Add a few cc. of hot cone, nitric acid, and allow to stand in the 
cold for about 10 minutes. Add 50 cc. more of hot cone, nitric 
acid, taking care to wash down the sides of the beaker; heat on 
the steam bath for at least an hour. While hot, decant the liquid 
through a Gooch containing a thick pad of asbestos, taking care 
to keep the insoluble material completely in the beaker. Wash 
with hot nitric acid, and suck dry. Empty the filter flask. Wash 
the insoluble residue with acetone, and then with a mixture of 
equal parts of acetone and chloroform, until the filtrate is color- 
less. The insoluble matter, which has been carefully retained in 
the beaker, is digested on the steam bath for 30 minutes with 35 
cc. of a 25% solution of sodium hydroxide. Dilute to 60 cc. with 
hot water, filter the solution, and wash with a hot 15% solution 
of sodium hydroxide. Test for the presence of lead by running 
some warm ammonium acetate solution containing an excess of 
the hydroxide through the pad into sodium chromate ; if a yellow 
precipitate is obtained, the pad must be washed until the wash- 
ings no longer give a precipitate with the sodium chromate 

11 Smith and Epstein, U. S. Bureau of Standards Tech. Paper, No. 136 ; J. Ind. 
Eng. Chem. 11, 33-6 (1919). 



MINERAL ANALYSIS 107 

solution. Next wash the residue a few times with hot cone. 
hydrochloric acid, and finally with warm 5% hydrochloric acid. 
Remove the crucible from the funnel, taking care that the outside 
is perfectly clean, and dry in an air bath at 150C to constant 
weight. Burn off the carbon at a dull red heat, cool and reweigh; 
the difference in weight is approximately 105% of the carbon 
originally present in the form of lampblack or gas black. 

Several points must be carefully watched during this pro- 
cedure: the acetone and hot nitric acid must not be brought to- 
gether, since they react with considerable violence. Again, care 
must be used in the alkali washing to avoid carrying through 
the filter some of the gas black; the pad must be unusually thick 
and free from channels. This is one of the principal reasons for 
keeping the fillers in the beaker until the last moment. 

The published data of Smith and Epstein show that the loss in 
weight on ignition is about 5% higher than the carbon a ctually 
present; hence the factor 105. The 5% is probably organic 
matter not removed by the preliminary steps of the method. 
Mineral rubber has no effect on the determination. Calcium sul- 
fate, if retained with the fillers, would be reduced during the 
ignition of the carbon, and would give high results for the latter. 
Quite apart from the point raised by Smith and Epstein that 
calcium sulfate is rarely found in rubber compounds, usually only 
when associated with antimony, the treatment with strong acids, 
and boiling, would suffice to dissolve out a considerable quantity 
of calcium sulfate, which is quite soluble in hot nitric or hydro- 
chloric acid solutions. 

Red Lead. The peroxide of lead contained in red lead is not a 
particularly desirable constituent for rubber compounds, and 
some specifications, notably those for 30 or 40% Para insulation, 
forbid its use. The Joint Rubber Insulation Committee 1Z gives 
the following test for red lead: Dissolve a 1 gr. sample, pre- 
viously extracted with acetone, in xylol; when the rubber has 
been completely dissolved, filter through a Gooch crucible, wash- 
ing thoroughly with benzol, alcohol and acetone. Transfer the 
Gooch pad to a distilling flask, add hydrochloric acid, and distil 
over the chlorine into a potassium iodide-starch solution. If 
more than 0.1 cc. of N/10 sodium thiosulfate is required to titrate 
the iodine liberated, red lead may be assumed to be present. 

"J. Ind. Eng. Chem. 6, 75-82 1914). 



108 THE ANALYSIS OF RUBBER 

This method was suggested for insulation compounds, and, as 
far as it has been tested, has given satisfactory results. The 
method depends upon the liberation of chlorine by the action of 
the peroxide on the hydrochloric acid. Some off-color litharge 
samples have given positive tests under this method; which is 
what we might expect, since these lots contain a greater amount 
of peroxide than they should, and yet not enough to be classed 
as red lead. They are really mixtures of red lead and litharge, 
and should be so treated. 

Chromates, such as chrome yellow, will give this reaction, but 
they should cause no confusion, since the color of the sample will 
usually tell whether chromates are present. It would be unusual 
indeed to have both chromates and lead peroxide present in the 
same sample. 

Chromates. While chromium is not a frequent constituent of 
rubber goods, it is a possibility, and should be determined. There 
is considerable analogy between the analyses of the pigments in 
printing inks, and those in rubber compounds, and the following 
method, originally used in the analysis of printing inks, should 
be equally available for rubber compounds. 

Fuse 0.500 gr. of rubber with equal parts of sodium peroxide 
and potassium carbonate, using a nickel crucible. The heating 
must proceed cautiously until the organic matter is destroyed, 
after which the melt can be heated strongly for 10 or 15 minutes. 
Cool, extract with water, and filter. The chromium is in the fil- 
trate as chromate. Pass carbon dioxide through the filtrate, and 
heat on the steam bath, in order to precipitate any lead which 
may be held up by the caustic alkali; filter if necessary. Cool, 
acidify strongly with hydrochloric acid, add potassium iodide 
and starch solution, and titrate with standard N/10 sodium 
thiosulfate to a colorless solution. The solution may be stand- 
ardized against potassium bichromate, and the chromium calcu- 
lated to CrO s , in which condition it no doubt exists in the com- 
pound. 

This method has been found simple and accurate in the pres- 
ence of lead, manganese, clay, and other fillers likely to be 
present in printing ink*, and should be fully as satisfactory for 
rubber goods. 

Glue. Make a qualitative test as follows: Digest 1 gr. in 
cresol, or xylol (any solvent for rubber which does not attack 



MINERAL ANALYSIS 109 

glue will do just as well) until the rubber is decomposed. Dilute 
with petroleum ether, and filter through filter paper. Wash the 
residue with alcohol, and after allowing the alcohol to evaporate 
wash the residue back into a beaker, cover with water, and boil. 
Filter off the insoluble, and test the filtrate for glue with a solu- 
tion of tannic acid. Traces of glue will give only a milky cloudi- 
ness, but with large quantities a heavy precipitate is thrown 
down. 13 

The quantitative determination of glue is based on the deter- 
mination of nitrogen by the Kjeldahl method. This procedure 
assumes that the principal source of nitrogen, the organic accel- 
erators, will be removed during the acetone extract. The U. S. 
Bureau of Standards extracts with the mixed solvents, 68% by 
volume of chloroform, and 32% of acetone. From this point on, 
the procedures are alike: the dried sample is heated with sulfuric 
acid, potassium or sodium sulfate and a small amount of copper 
sulfate, the clear solution is diluted, made alkaline, and the 
ammonia distilled into a standard solution of N/10 sulfuric acid. 

Practically every one is agreed that the Kjeldahl method is 
the most satisfactory means of approach in the quantitative de- 
termination of glue, but differences arise as to the factor by which 
to calculate from nitrogen to glue. The Bureau of Standards 
uses 5.56; others prefer the factor of 6.25. Since glue is not a 
pure chemical substance, we are bound to have differences of 
opinion, but the weight of evidence seems to lean towards the 
higher factor. The collagens have 17.9% of nitrogen, and even 
assuming that in glue we have reasonably pure collagens, we must 
take into consideration the water which is always present, and 
which will average about 10%, hence, in the collagens, 5.56 
would be the correct figure, and calculating that this is only 90% 
of the whole, we get 6.18 as the corrected figure. 

Another variable will be the amount of nitrogen in the in- 
soluble matter in the rubber, which, as we have already discussed, 
may run from 2 to 6% of the rubber, and may contain from 10 
to 18% of nitrogen. 

In view of the above facts, it is obvious that any factor will 

]3 A much shorter method of testing qualitatively for glue is as follows: 
Heat 5 to 10 gr. finely divided sample of rubber with 25 cc. of water for 2 to 4 
hours; decant, and test for glue with 2 or 3 ce. of a solution of tannic acid. 
This test is not as safe as the one given above ; glue to the extent of 2 or 3% 
may be easily overlooked, and hence the method is not recommended. 



110 THE ANALYSIS OF RUBBER 

at best give only an approximation of the truth, but, even so, it 
is believed that better results on the average will come from 
the use of the factor 6.25, and which we recommend. 

Ground Organic Wastes. In a mixture of rubber and wastes, 
containing such materials as leather, cork, wool, silk, cotton or 
other vegetable fibre, etc., the separate determination of these 
wastes is usually of no consequence, and a direct determination 
of the rubber by the nitrosite method will give practically all the 
information that one really needs. Some of the solvents, such as 
xylol, cymene, and possibly others, will determine the rubber 
accurately enough in the presence of such materials. 

There are occasions when we may be called upon to determine 
cotton, as for example, in balloon fabrics, where it is difficult to 
separate the rubber from the cotton, etc. For this purpose, the 
method of Epstein and Moore 14 will suffice: 

Treat a 0.500 gr. sample of the rubber with 25 cc. of freshly 
distilled cresol (b.p.l98C) for 4 hours at 165C. Cool, add 
200 cc. of petroleum ether very slowly, and with constant agita- 
tion. Filter through a Gooch, and wash with petroleum ether, 
then with hot benzene, and finally with acetone. Add hot 10% 
hydrochloric acid, and transfer the contents of the flask to the 
Gooch, and wash at least ten times with hot acid. Wash free of 
chlorides, and then with acetone until the filtrate is colorless. 
Wash with a mixture of equal parts of acetone and carbon 
bisulfide. Wash with alcohol, and dry for V/% hours at 105C. 
Transfer the asbestos pad and fillers to a weighing bottle, dry 
for about 10 minutes further, cool and weigh. 

Transfer the contents of the weighing bottle to a 50 cc. beaker 
and pour over it 15 cc. of acetic anhydride and 1 to 2 cc. of cone, 
sulfuric acid, and digest on the steam bath for one hour. Cool, 
dilute with 25 cc. 90% acetic acid, and filter through a weighed 
Gooch. Wash with hot 90%, acetic acid until the filtrate is color- 
less, and then four times more. Wash about 5 times with acetone, 
remove the crucible from the funnel, and dry to constant weight 
at 150C. The cellulose has been dissolved out, and the usual 
calculations are made. 

Sponge Rubber. One of the interesting points in connection 
with the analysis of sponge rubber is to determine the substance 

14 U. S. Bureau of Standards Tech. Paper, 154; The Rubber Age, 6, 289 93 
(1920). 



MINERAL ANALYSIS 111 

used to produce porosity. Organic liquids, if used, will have been 
dissipated by the time the sample reaches the analyst. If either 
ammonium carbonate or sodium carbonate has been used suf- 
ficient material will usually remain to give a qualitative test, 
although a quantitative determination is out of the question. 

Grind the sample into small particles, being particularly care- 
ful to avoid heating. Digest 10 gr. of the sample in 25 cc. of 
water for one hour, and filter. Divide into two portions; into 
the first, add 10 cc. of 20% caustic soda, and note any odor of 
ammonia which may escape. A positive test indicates ammo- 
nium carbonate. A more delicate test may be made by adding 
a little hydrochloric acid, and evaporating to dryness, and treat- 
ing the dried residue with a small amount of strong alkali. Evap- 
orate the second portion of the extract to dryness, take up with 
25 cc. of water, and add a few drops of methyl orange. Titrate 
with N/10 hydrochloric acid; any appreciable quantity of alka- 
line carbonate, in the absence of ammonia, will be a fair indica- 
tion that sodium bicarbonate was used. In case ammonium car- 
bonate was used, the residue from the second filtrate should be 
heated strongly to remove the ammonia, and thus determine 
whether both substances were used. 

Negative tests for both ammonium carbonate and sodium bi- 
carbonate may be taken to indicate that organic liquids have 
been employed. 

Specific Gravity. 

Rubber Compounds. For ordinary rough work, where great 
accuracy is not necessary, and when pieces of from 2 to 5 gr. are 
available, Young's gravitometer is a rapid and convenient instru- 
ment. When the bearings are clean, and the instrument in good 
working order, the results are usually with 0.02, plus or minus 
and are frequently only half that. 

For greater accuracy, the pyenometer is the best thing to use. 
Weigh out about 5 gr. in small strips, place them in the pyenom- 
eter bottle, and fill with distilled water to the mark, being 
careful that no bubbles adhere to the rubber, and then weigh. 
Knowing the weight of the bottle filled with water, the weight 
of water displaced by the rubber is easily calculated, and from 
this, the specific gravity of the rubber. Ordinarily the specific 
gravity is expressed to two decimal places, but even without 



112 THE ANALYSIS OF RUBBER 

bringing the pycnometer to constant temperature 2 the calculations 
may be made to the third decimal. 

It has been found convenient, both in using Young's gravi- 
tometer and the pycnometer, to wet the rubber with a soap solu- 
tion, brushing it on with a camel's hair brush, and then rinsing 
the rubber with distilled water. It eliminates the risk of air 
bubbles, and does not affect the accuracy of the determination. 

Pigments and Fillers. Pigments in lumps may be handled as 
in the case of rubber compounds; the pycnometer is probably 
better for the purpose. 

Oils are determined with the Westphal balance, or, for quicker 
and less accurate work, a hydrometer will do. 

For powders, or small particles, the pycnometer is required. 
The liquid chosen must be such as to have no effect on the pig- 
ment being tested. For many of them, water will answer, but 
where this is impossible, any other liquid will do just as well, 
providing it does not react with, or dissolve the pigment. With 
liquids other than water, the coefficient of expansion may be 
such as to make it imperative to hold to a standard temperature 
of say 25C, the specific gravity being referred to water at that 
temperature. 

Weigh out 5 gr. of the pigment, transfer to a pycnometer, and 
fill the latter about two-thirds full. Boil the liquid for 10 to 15 
minutes, and then place under a vacuum bell jar. When the air 
has been entirely removed from the sample, cool to room tem- 
perature (or to a standard temperature of 25C), fill up to the 
mark, and weigh. When a liquid other than water is used, deter- 
mine its specific gravity as referred to water at 25C, and use 
this to calculate the gravity of the pigment. 

Reclaimed Rubber. One of the important values connected 
with reclaimed rubber is its gravity, and yet it is frequently so 
porous that ordinary methods fail to secure accurate results. 
In thin sheets, and with boiling water, fair results may be ob- 
tained. W T hen a small mixing mill has been available, the 
following scheme has been found eminently satisfactory: 

Mix 450 grams of reclaimed rubber and 50 grams of sulfur, 
until thoroughly and evenly mixed. The total weight of the 
batch after mixing should be within 1 gr. of 500. Vulcanize a 
small strip from the mix, and from this strip determine the 
specific gravity of the mixture, Calculate the specific gravity of 



MINERAL ANALYSIS 113 

the reclaimed rubber, taking the specific gravity of sulfur as 2.0. 
If the specific gravity of the mixture is a, and the specific gravity 
of the reclaim x, the calculation is as follows: 

100a — 20.00 



x = 



90 



For example, if the gravity of a mixture is 1.370, the calcula- 
tion would be: 

__ 100 X 1-370 — 20.00 117.00 
X ~" 90. " 90.00 

x = 1.30 

A chart can be drawn, so that given the specific gravity of a 
mixture that of the reclaimed can be read off directly. A differ- 
ent mixture of reclaimed rubber and sulfur may be employed, 
making the necessary alterations in the formula, the only requi- 
site being that there should be sufficient sulfur for vulcanization. 



Chapter XI. 
Microsectioning and Micro-photography. 

Microphotographs of rubber goods have been known for a 
number of years, Weber showing some excellent photographs of 
hard and soft rubber goods in his book on India Rubber. Re- 
cently, there has been considerable attention paid to the use 
of the microscope in mineral analysis of small amounts of 
materials, and in the examination of commercial materials, mix- 
tures, etc. It has been realized that the chemical analysis does 
not give the last word, and that frequently the difference in 
the properties of two materials may be a matter of their physical 
state, rather than their average chemical composition. In the 
rubber industry, many laboratories have been working along 
the lines of preparing sections of rubber compounds thin enough 
to be examined under transmitted light, instead of reflected light, 
as had been so largely the practice. The problem very quickly 
narrowed itself down to a question of mechanical manipulation, 
for even the crude sections first prepared showed that the pro- 
cedure was feasible, and that information could be obtained not 
only regarding composition, but even the properties of rubber 
compounds, if the proper sections could be prepared. 

The microsectioning has largely been done with the Spencer 
microtome, which seems adequate for the purpose. The main 
difficulty has been to so stiffen the rubber compound that it 
would have no motion when being cut. Freezing was resorted 
to, the earliest attempts employing the expansion of carbon 
dioxide directly on the stage of the microtome, or surrounding 
the specimen to be cut with solid carbon dioxide. Further 
stiffening of the rubber was obtained by imbedding it in such 
materials as starch paste, water-glycerine solutions, paraffin, etc. 
The best results are obtained with material which does not 
become brittle at the low temperatures employed. Even carbon 
dioxide cooling was found to be insufficient for the purpose, and 
the use of liquid air was resorted to, with eminently satisfactory 

114 



MICROSECTIONING AND MICROPHOTOGRAPHY 115 

results. Sections thinner than lj« are now being prepared, a great 
deal of work has been started, and we are beginning to see the 
fruits of this work. 

Liquid air is probably not available for many laboratories, 
but in such cases the use of carbon dioxide alone will be found 
to give results well worth the effort, even though better could 
be obtained with the cooling effected by the liquid air. 

Perhaps one of the most interesting points brought out by this 
new phase of rubber testing came to light at the meeting of 
the American Chemical Society at Rochester, in April, 1921. 
Schippel x had previously shown by experiment that compounded 
and vulcanized rubbers showed an increase in volume on stretch- 
ing, and his explanation was that vacu were formed around the 
mineral particles, caused by the rubber being pulled away from 
the surface of the pigment. Green 2 exhibited some microphoto- 
graphs of sections of rubber under strain, wherein the vacu 
caused by the rubber leaving the surface of the pigment were 
clearly visible. Still more important was the evident fact that 
only the larger or coarser particles showed this phenomenon. 
The mechanism of tearing, rapid wear, etc., when coarse pig- 
ments are used, was quite apparent. Green's work reflects credit 
on the soundness of Schippel's reasoning. 

The work of Breyer and his coworkers Ruby, Depew, and 
Green, and of I. C. Diner, should shortly put us in a position 
where we can take a piece of rubber and at least qualitatively 
tell what pigments are present. It is too much to expect any- 
thing in the quantitative line, especially when one considers the 
extremely small area covered by these microphotographs, and 
the difficulty of securing even mixing of a plastic such as rubber 
with dry fillers. We know that we have variations in compo- 
sition from one part of a batch to another; and this variation 
must be very much greater when the sample under observation 
weighs less* than a milligram. It is quite within the range of 
probability that we shall, by careful sectioning, be able to tell 
whether we are dealing with carbon black or lamp black; and 
particularly identify such substances as Tripoli, aluminum flake, 
talc, asbestine, etc., in mixtures of two or more, under which 

'J. Ind. Eng. Chem. IS, 33-7 (1920). 

1 Henry Green. "Volume increase of compounded rubber under strain," Rub- 
ber Division, American Chemical Society, Rochester, April, 1921. 



116 THE ANALYSIS OF RUBBER 

conditions the identification by chemical or mechanical means is 
practically impossible. 

The general scheme for the examination of microsections 3 deals 
with (a) reflected light; (b) transmitted light ; (c) polarized light. 
With reflected light, we use not only vertical, but oblique rays, 
so as to get some idea of the surface, as well as the color of the 
section. In transmitted light we have a new color classification, 
wherein some fillers which are opaque and colored in reflected 
light may be translucent and show a different color by trans- 
mitted light. In polarized light, we have the differences in opti- 
cal behavior between crystalline and non-crystalline substances; 
interference figures, extinction angles, etc., to further classify the 
materials under observation. Considering the comparatively 
limited number of substances one finds in rubber compounds, as 
compared with the entire mineral field, the possibility of exact 
identification is very great. 

As far as the identification of fillers is concerned, the future 
seems bright, and today practically all the work is being con- 
ducted along these lines. We have still to consider the possibility 
of identifying different rubbers, or the rubber plastics, such as 
the mineral rubbers, substitutes, etc., reclaimed rubber, soften- 
ing oils and waxes, etc. For some of these substances, notably 
mineral rubber, paraffin, rosin, oil substitutes, we have excellent 
chemical means of identification, and more or less accurate means 
for their quantitative determination. The problem of the iden- 
tification of reclaimed rubber, and the different grades of new 
rubber, is still open for solution, and it may be that this new 
means of research will prove of valuable assistance in investiga- 
tions of this sort. 

8 Some excellent text books for this type of work are found in "Minerals in 
Rock Sections," by Luquer, D. Van Nostrand Co., and "Characters of Crystals," 
by A. J. Moses, D. Van Nostrand Co. The preparation and identification of 
minerals in rock sections, measurement of crystal faces, extinction angles, lines 
of cleavage, etc., will be excellent, and withal comparatively simple preparation 
for the study of microsections of rubber. Fred E. Wright (see bibliography) 
has done some excellent work in the field of the identification of minerals in 
rocks, through the aid of the petrographic microscope, and any one attempting 
work in the field of the microscopic examination of rubber compounds will find 
a careful study of Wright's work to be of great help. 



Chapter XII. 
Calculation to Approximate Formulas. 

The greater number of analyses are made for the purposes of 
checking factory production, and for comparing finished goods 
sold under chemical specifications. In such cases, a complete 
analysis is seldom desired; for factory purposes, a few deter- 
minations suffice, and for specification purposes the analysis is 
carried just far enough to decide whether or not the specifications 
have been complied with. In the latter case, it is usually suf- 
ficient to report the percentage of the rubber present, the pres- 
ence or absence of reclaimed rubber, the free, total, and barium 
sulfate-sulfur, the presence and approximate amounts of oils, 
waxes, mineral rubbers, substitutes, and any other organic fillers 
likely to have a bearing on the analysis. 

There are times when one is interested in learning everything 
concerning an article, and then, in addition to the foregoing, we 
need a complete analysis of the mineral fillers, both as to the 
basic and acidic radicles. From these data, we build up an 
approximate formula. The report of the analysis should cover 
the following points: 
Rubber hydrocarbons 
Acetone extract, sulfur free 

Color and appearance of extract 
Saponifiable matter 
Unsaponifiable matter 
Mineral hydrocarbons 
Vegetable hydrocarbons 
Chloroform extract 

Color and appearance of extract 
Alcoholic potash extract 

Color and appearance of extract 
Total sulfur 
Free sulfur 
Sulfur of Vulcanization 

117 



118 THE ANALYSIS OF RUBBER 

Glue 
Carbon 

Other organic fillers 
Mineral Fillers 
Bases 
Aluminium as A1 2 3 
Antimony as Sb 2 S 8 
Barium as BaO 
Calcium as CaO 
Iron as Fe 2 8 
Lead as PbO 
Magnesium as MgO 
Zinc as ZnO 
Any other bases 
Acids 

Carbonate as C0 2 
Silica as Si0 2 
Sulfide-sulfur as S 
Sulfite-sulfur as S0 2 
Sulfate-sulfur as S0 3 
Organic Accelerators 
Specific Gravity 

With these data before us, we may proceed with the recon- 
struction of the compound. 

Rubber. The rubber is the sum of the rubber hydrocarbons 
(sulfur free), and the acetone, chloroform and alcoholic potash 
extracts, providing that no organic matter, other than that 
originally present in the rubber, is shown by the analyses. 
Ordinarily, with new rubber, the acetone extract will not exceed 
4%, the chloroform extract in a properly cured article 2%, and 
the alcoholic potash extract 1%, based upon the rubber. If any 
appreciable quantity in excess of these amounts is found, it must 
be explained. 

Sulfur. The sulfur added as such is the sum of the free sulfur 
and the sulfur of vulcanization, plus any sulfur which may have 
combined with the fillers during vulcanization. This latter item 
is often difficult, and sometimes impossible to determine, but a 
knowledge of the general procedure in designing rubber com- 
pounds will be a help. 

Organic Fillers. The oils, fats, waxes, etc., are determined 



CALCULATION TO APPROXIMATE FORMULAS 119 

from tests on the acetone, chloroform and alcoholic potash ex- 
tracts. Mineral rubber at best can be only approximated. 
Special fillers, such as glue, cellulose, carbon, etc., are set down 
just as they are determined. 

Inorganic Fillers. With a knowledge of what bases and acids 
are present, we may start to build up the composition of the 
mineral fillers. 

Antimony Compounds. If only antimony, sulfur and calcium 
sulfate are found, in addition to the rubber, we know that we 
have a mixture of golden sulfide and rubber, and not only is 
the calculation simple, but also we have the formula of the 
golden sulfide used. 

Barium. All barium should be calculated to sulfate, unless 
by analysis barium carbonate is shown to be present. 

Calcium. In the absence of antimony, calcium may be calcu- 
lated to the carbonate, unless the quantity present is less than 
1%. In such cases, especially in the absence of reclaimed rubber, 
it may be assumed, with some assurance, that this small amount 
was added as hydrated lime. In the presence of whiting, the 
hydrated lime cannot be detected. 

Aluminium. Aluminium is probably present as a silicate. The 
microscope will be found to be an absolute necessity to determine 
which silicate is present. In the absence of magnesium a white 
compound will probably contain aluminum flake, or white clay. 
Some clays contain titanium, and a positive qualitative test for 
titanium would be sufficient indication that the substance is 
clay. Titanium oxide, associated with barium sulfate, is used 
as a paint pigment, but only in an experimental way in rubber. 

Iron. Iron is usually present as the oxide, but frequently is 
associated with clay. It is sufficient for the purpose to report 
the oxide and clay separately; then, in rebuilding the compound, 
any clay in excess of that found in the iron oxide used must be 
added as such. 

Lead. Without question, lead is one of the most difficult sub- 
stances to work upon. If organic accelerators are present, it is 
probable that lead oleate, sublimed white or blue lead is present. 
Probably as safe a thing to do as any is to work up all of the 
other fillers first, and then apply any sulfide, sulfite, or sulfate- 
sulfur to the lead. 

Magnesia. Magnesia may be present in one of three forms, the 



120 THE ANALYSIS OF RUBBER 

oxide, carbonate, or silicate. With silica present, and no 
aluminium, a magnesium silicate is probable. In the absence of 
whiting, any carbon dioxide found is probably combined with 
magnesium, although lead carbonate (white lead) may interfere. 
The specific gravity of the compound as a whole is one means 
for distinguishing between the oxide and carbonate. 

Zinc. Zinc is usually present as the oxide, and the simul- 
taneous presence of barytes is not evidence that lithopone is 
present. In the absence of lead and antimony, any sulfide-sulfur 
will undoubtedly be combined with zinc. It is best to calculate 
all zinc as the oxide, and not to assume that lithopone is present 
unless there is an excess of sulfide-sulfur over that required for 
lead or antimony. 

After the approximate amount of the probable ingredients of 
the compound have been worked out as above, the sum should be 
in the neighborhood of 100% — if anything, should exceed that. 
The next step is to take this formula and calculate the specific 
gravity, which should check within 0.02 the specific gravity of 
the original compound. Any greater discrepancy than this re- 
veals some error, which must be checked up. Obviously, if our 
calculations are low, the high gravity substances are in error, 
and vice versa. If the gravities agree closely, then the figures 
may be rounded off to even percentages, to the nearest 0.25%, 
and brought by adjustment exactly to 100%. 

It must be very clear to every one that the interpretation of 
analytical results is a matter requiring experience, ingenuity, and 
a great deal of common sense. The intent of the above is cer- 
tainly not to lay down exact rules, but merely to indicate the 
general line of thought, permitting the analyst, with his first-hand 
information as to the progress of the analysis, to make such 
deductions as may seem wise. 



BIBLIOGRAPHY 



Allen and Johnston 

1. The exact determination of 

sulfur in soluble sulfates. J. 
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(1910). 
Paul Alexander 

2. Determination of sulfur in rub- 

ber. Gummi Ztg. 18, 729; 
Z. Angew. Chem. 17, 1799 
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3. Weber's method for the direct 

determination of rubber. 
Gummi Ztg. IS, 789-91 
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28, 765. 

4. Rubber nitrosite, and its use 

for the analysis of crude 
rubber and rubber products. 
Ber. 38, 181-4 (1905). 

5. Nitrosites of India rubber and 

their application to the 
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Angew. Chem. 24, 680. 

6. The desulfurization of vulcan- 

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Anon. 

7. Determination of sulfur in 

golden antimony sulfide. 
Chem. Ztg. 28, 595 (1904). 

8. Propositions for a uniform 

execution of tests in the 
evaluation of rubber. Caout- 
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5011-23 (1911). 

9. Sulfur chloride substitute and 

hot vulcanization. Gummi 
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10. The quantitative determina- 

tion of golden sulfide of 
antimony. Gummi Ztg. 29, 
137-9 (1914). 

11. Specifications and methods of 

analysis for mixtures con- 
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12. A study of factice and its 

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13. The use of jar rings containing 

lead oxide. Gummi Ztg. SO, 
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15. A brief review of the organic 

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16. The Bayer patent vulcaniza- 

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18. Electrolytic methods for de- 

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19. Water Extract of raw Rubber. 

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20. The Peachey vulcanization 

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21. Magnesium carbonate as a 

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22. Determination of sulfur in 

organic compounds. Z. 
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L. Archbutt 

23. Preparation of rubber for 

analysis. Analyst 38, 550-4. 
Austerweil 

24. Passage of hydrogen through 

rubber walls of balloons. 
C. R. 154, 196. 
S. Axelrod 

25. A method for the direct deter- 

mination of rubber in vul- 
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Gummi Ztg. 21, 1229 (1907). 



121 



122 



THE ANALYSIS OF RUBBER 



26. Direct determination of rub- 

ber in soft cured rubber 
goods. Chem. Ztg. 33, 895 
(1909). 
G. Ban 

27. Rubberized balloon fabrics. 

The Rubber Ind., 259 (1914;. 

28. Proofing airship fabric. Rub- 

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S. Bary 

29. Estimation of free sulfur in 

vulcanized rubber. Rev. 
gen. chim. 16, 142-5. 
Charles Baskerville 

30. Note on the preparation of 

rubber samples for analysis. 
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31. Examination of rubber tires. 

Chem. News 96, 2488. 

32. Analyses of vulcanized rub- 

ber goods. Analyst 85, 11- 
16. 

33. Some analyses of Hevea latex. 

Analyst 86, 6-9. 

34. Influence of mineral ingredi- 

ents on properties of rubber. 
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35. The nitrogenous constituent 

of Para rubber and its bear- 
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36. Determination of the insoluble 

in raw rubber. Analyst 37, 
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37. Testing of crude rubber. 

Caoutchouc & Guttapercha 
9, 6296-300 (1912). 

38. New extraction apparatus. 

Gummi Ztg. 27, 2087. 
R. Becker 

39. Determination of mineral rub- 

ber and similar products in 
rubber goods. Gummi Ztg. 
25, 598; Chem. Ztg. 85, 
288. 

40. Discussion of Hubener's tetra- 

bromide method. Gummi 
Ztg. 25, 531; 677 (1911); 26, 
1503 (1912). 
C. W. Bedford and Winfield Scott 

41. Reactions of accelerators dur- 

ing vulcanization. J. Ind. 
Eng. Chem. 12, 31-3 (1920). 
C. W. Bedford and L. B. Sebrell 

42. Reactions of accelerators dur- 

ing Vulcanization. III. 



Carbo-sulfhydryl accelera- 
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oxide. J. Ind. Eng. Chem. 
18, 1034-8 (1921). 

43. Reactions of accelerators dur- 

ing vulcanization. IV. 
Mechanism of the action of 
zinc compounds. J. Ind. 
Eng. Chem. 14, 25-31 (1922). 
Bedin 

44. Analysis of manufactured soft 

and hard rubbers. Ann. 
chim. anal. 23, 57-9 (1918). 
R. W. Belfit 

45. A method for the direct deter- 

mination of rubber in a 
compound. J. Ind. Eng. 
Chem. 8, 326-7 (1916). 
Gustav Bernstein 

46. Contribution to the study of 

the cold vulcanization of 
rubber. Z. Chem. Ind. Kol- 
loide 11, 185. 
John M. Bierer 

47. Crimson antimony. India 

Rubber World 63, 17-8 
(1920). 
Jules Bock 

48. Determination of crude rub- 

ber. Rev. Gen. Chim. 14, 
209-21 (1911). 
J. Boes 

49. The investigation of rubber 

goods. Apoth. Ztg. 22, 1105. 
C. R. Boggs 

50. Direct determination of rub- 

ber. 8th Int. Cong. App. 
Chem. 9, 45-58. 

51. Vulcanization of rubber by 

selenium. J. Ind. Eng. 
Chem. 10, 117-8 (1918). 
L. M. Bourne 

52. Resin and sulfur in India rub- 

ber. Chem. Eng. 6, 195. 
Jean Boutaric 

53. Analysis of rubberized fabrics. 

Caoutchouc & Guttapercha 
17, 10202-6. 
Britland and Potts 

54. Use of pyridine in rubber 

analysis. J. Soc. Chem. Ind. 
29, 1142. 

55. Ceresin wax in rubber mixings. 

India Rubber J. 43, 333. 
Bruggeman 

56. Rapid determination of fillers 

in rubber compounds. 
Gummi Ztg. 25, 1529; J. Soc. 
Chem. Ind. SO, 908. 



BIBLIOGRAPHY 



123 



G. Bruni 

57. Solubility of crystalline sub- 

stances in rubber. Giorn. 
chim. ind. appl., Feb., 1921. 
G. Bruni and C. Pelizzola 

58. The presence of manganese in 

raw rubber and the origin of 
tackiness. India Rubber J. 
62, 101-2 (1921). 

59. The tackiness of raw rubber 

and the aging of vulcanized 
rubber goods. India Rubber 
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G. Bruni and E. Romani 

60. Mechanism of action of cer- 

tain accelerators of vulcan- 
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62,63-6 (1921). 
T. Budde 

61. The determination of true 

rubber in rubber goods. 
Chem. Zentr. II, 173 (1905). 

62. Determination of rubber in 

cold cured rubber. Gummi 
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63. Determination of rubber as 

tetrabromide. Gummi Ztg. 

22, 333 (1908). 

64. The determination of vulcan- 

ized rubber. Apoth. Ztg. 23, 
318. 

65. The valuation of cold vulcan- 

ized rubber by the tetrabro- 
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24, 529. 

66. New method for determining 

combined sulfur in vulcan- 
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23, 1143; 24, 4-6 (1909); 25, 
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67. Estimation of rubber as tetra- 

bromide. Z. Angew. Chem. 

24, 954 (1911). 
E. Bunschoten 

68. Vulcanization with sulfur ac- 

cording to Ostromuislenskii. 
Chem. Weekblad 15, 257-68 
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69. The testing of rubber goods. 

Circular 38. Fourth edition, 
1922. 
E. M. Camerman 

70. Analysis of manufactured rub- 

ber. Eng. News 56, 551. 
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71. Bromination of vulcanized rub- 

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Caspari and Porritt 

72. The theory of vulcanization. 

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73. General scheme for the analy- 

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74. The direct determination of 

the sulfur of vulcanization. 
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75. Analysis of rubber goods con- 

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Rochester meeting of the 
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Scherrer 

76. Determination of antimony in 

rubber goods. Rubber Age 
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David S. Collins 

77. Infusorial Tripoli. Rubber 

Age 5, 101 (1919). 
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78. Effect of organic accelerators 

on the vulcanization coeffi- 
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Benton Dales and W. W. Evans 

79. The use of the microscope and 

photomicrographs in the 
study of inorganic materials 
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80. Coal tar products used in the 

rubber industry. Color Trade 
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81. Solvents and thinners used in 

the rubber industry. India 
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82. Oils, fats, waxes and resins 

used in the rubber industry. 
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563-6 (1921). 

83. Carbons and hydrocarbons 

used in the rubber industry. 
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84. Pitch hydrocarbons used in the 

rubber industry. India Rub- 



124 



THE ANALYSIS OF RUBBER 



ber World 64, 821-4 
(1921). 

85. The action of volatile organic 

solvents and vulcanizing 
agents on organic compound- 
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gums. New York meeting 
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86. Defects in industrial rubber 

goods. Met. Chem. Eng. 18, 
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87. Rubber and Jelutong. Met. 

Chem. Eng. 18, 296-8 (1918). 

88. The rubber industry as a user 

of dyes and coal tar products. 
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Frederic Dannerth and R. M. Gage 

89. A method for the valuation of 

washed and dried rubber. 
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E. R. Darling 

90. Determination of combined 

sulfur in sulfur chlorides. 
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91. The determination of sulfur 

in rubber. Chem. Analyst 
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92. A rapid volumetric method for 

the estimation of free sulfur. 
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93. Estimation of the content of 

unsaponifiable resins in vari- 
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94. The determination of sulfur in 

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95. The determination of substi- 

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96. Report of the Joint Rub- 

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98. Some microsections cut from 

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99. Quantitative determination of 

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100. On the determination of rub- 

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101. Microscopy of rubber fillers. 

New York meeting of the 
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102. New methods of analysis of 

raw rubber. Gummi Ztg. 20, 
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103. Relation between specific grav- 

ity and the sulfur content of 
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104. Effect of heavy magnesia as a 

filler upon India Rubber. 
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105. Influence of light magnesia as 

a filler upon India Rubber. 
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(1906). 

106. Vulcanization of India rubber 

in the presence of litharge. 
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107. Influence of pressure on rate 

of vulcanization, strength, 
and oxidation of different 
kinds of rubber. Chem. Ztg. 
81, 638-9 (1907). 

108. The influence of zinc oxide on 

the vulcanization of, and 
oxidation of rubber. Gummi 
Ztg. 21, 5 (1907). 

109. The melting points of some 

rubbers. Gummi Ztg. 21, 
670 (1907). 

110. The determination of sulfur 

in vulcanized India rubber 
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Dennstedt's method. Gummi 
Ztg. 21, 497 (1907). 

111. The dyeing of rubber with or- 

ganic dyes. Chem. Ztg. 37, 
1162. 



BIBLIOGRAPHY 



125 



112. Vulcanization catalysts. Gum- 

mi Ztg. 29, 424-6 (1915). 
Rudolf Ditmar and O. Dinglinger 

vulcanization and oxidation 
of rubber. Gurnmi Ztg. 

113. Effect of powdered glass on 

2./, 234-5 (1907). 
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114. Changes occurring in the most 

important inorganic fillers 
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Roll. Z. 11, 77-80. 
E. D. Donaldson 

115. Rapid electrolytic method for 

total lead and zinc in rubber 
compounds. Chem. Analvst 
15, 11-12 (1915) ; India Rub- 
ber J. 57, 1100 (1919). 
Andre Dubosc 

116. Action of ozone and oxozone 

in the analysis of rubber. 
Caoutchouc & Guttapercha 
10, 7105 (1913). 

117. Analysis of vulcanized rubber. 

Caoutchouc & Guttapercha 
13, 8782-3 (1916). 

118. Method of determination and 

identification of proteins in 
rubber. Caoutchouc & Gut- 
tapercha 13, 8810-1 (1916). 

119. The analysis of rubber ma- 

terials. Caoutchouc & 
Guttapercha IS; 8939-44; 
8980-3 (1916). 

120. The role of analysis in the 

manufacture of rubber 
goods. Caoutchouc Sz Gut- 
tapercha 13, 9055 (1916). 

121. Action of amines in vulcaniza- 

tion. Caoutchouc <fc Gutta- 
percha, 13, 9064 (1916). 

122. Changes in resin content of 

rubber on vulcanization. 
Caoutchouc & Guttapercha 

13, 9094-5 (1916). 

123. Physical and chemical analysis 

of rubber thread. Caout- 
chouc & Guttapercha 13, 
9007-8 (1916). 

124. Analysis of vulcanized rubber. 

Caoutchouc & Guttapercha, 

14, 9189-91 (1917); 9213-6 
(1917); 9309-13 (1917). 

125. Application of catalysis to 

vulcanization. Rubber Age 
3, 78-9 (1918). 

126. A comparison of the effects 

produced by organic accel- 
erators in the vulcanization 



of rubber. Caoutchouc & 
Guttapercha 15, 9635-7 
(1918). 

127. Rubber substitutes, or vulcan- 

ized oils. Chimie & Indus- 
trie 1, 727-32 (1918). 

128. Polymerization and oxidation 

of crude rubber. Caoutchouc 
& Guttapercha 15, 9478-82 
(1918). 

129. Action of lipase on white fac- 

tice. Caoutchouc & Gutta- 
percha, 16, 9722-7 (1919). 

130. Vulcanization accelerators. 

Caoutchouc & Guttapercha 
16, 9853; 9856-65 (1919). 

131. Analysis of vulcanized rubber. 

Caoutchouc & Guttapercha 
16, 9900-7 (1919). 

132. Determination of sulfur exist- 

ing as sulfides in vulcanized 
rubber. Caoutchouc & Gut- 
tapercha 16, 9952-3 (1919). 

133. The use of furfural in rubber 

analysis and in the rubber 
industrv. Caoutchouc & 
Guttapercha 16, 9957-9 
(1919). 

134. Estimation of rubber and tex- 

tiles in impermeable fabrics. 
Caoutchouc & Guttapercha 
16, 9907-11 (1919). 

135. Analysis of vulcanized rubber. 

Caoutchouc & Guttapercha 
16, 9901-6 (1919). 

136. Analysis of crude rubber. 

Caoutchouc & Guttapercha 

16, 10051-3 (1919). 

137. The use of carbon black in 

tires. Caoutchouc & Gutta- 
percha 17, 10274-5 (1920). 

138. The discovery of accelerators. 

Caoutchouc & Guttapercha 

17, 10427 (1920). 

139. Theory of the acceleration of 

vulcanization. Caoutchouc 
& Guttapercha 17, 10511-4 
(1920). 

140. Application of some amides 

and amines of furfural to 
vulcanization. Caoutchouc 
& Guttapercha 17, 10495-505 
(1920). 

141. The new vulcanization and 

accelerators. I. Caoutchouc 
& Guttapercha 18, 11012-5 
(1921). 

142. Ibid. II. Caoutchouc & Gut- 

tapercha IS, 11121-4 (1921). 



126 



THE ANALYSIS OF RUBBER 



143. Chemical analysis of rubber 

articles. Ann. chim. anal, 
appl. 8, 335-44 (1921). 

144. The new vulcanization and ac- 

celerators. Caoutchouc <fe 
Guttapercha 19, 11171-6 
(1922). 
Andre Dubosc and Jean Wavelet 

145. Mineral Rubber. Caoutchouc 

& Guttapercha 16, 10037-40 
(1919). 

146. Action of lipases on oils vul- 

canized with sulfur chloride 
(white substitutes). Bull. 
Soc. Ind. Rouen 47, 47-59 
(1919). 
W . A. Ducca 

147. The determination of rubber 

as a tetrabromide. J. Ind. 
Eng. Chem. 4, 372-4 (1912). 
Richard B. Earle 

148. Report of Committee on or- 

ganic accelerators, Rubber 
Section, American Chemical 
Society. J. Ind. Eng. Chem. 
10, 865 (1918). 
B. ./. Eaton 

149. Vulcanization catalysts. Agr. 

Bull. Federated Malay States 
5, 38-43 (1916). 

150. The natural accelerators ol 

Para rubber. J. Soc. Chem. 
Ind. 37, 51T (1918). 
B. J. Eaton and F. W. Day 

151. The distribution of nitrogen 

in coagulum and serum of 
Hevea latex on coagulation 
with acetic acid. Agr. Bull. 
Federated Malay States 4, 
350-3 (1916). 

152. A preliminary investigation on 

the estimation of free and 
combined sulfur in vulcan- 
ized rubber, and on the rate 
of combination of sulfur with 
different types of plantation 
rubber. J. Soc. Chem. Ind. 
86, 16-20 (1917). 

153. Investigation on sulfur in vul- 

canized rubber. Agr. Bull. 
Federated Malay States 6, 
73-87 (1917). 
Junius David Edwards 

154. Methods of exposure and per- 

meability tests of balloon 
fabrics. Third Annual Re- 
port. Nat. Advisory Comm. 
for Aeronautics, 459-63 
(1917), 



H. A. Endres 

155. The relative activity of cer- 

tain accelerators In the vul- 
canization of rubber. Caout- 
chouc & Guttapercha 18, 
11089-97 (1921). 
S. W. Epstein and B. L. Gonyo 

156. The extraction of rubber 

goods. Rubber Age 6, 445- 
7 (1920). 
S. W. Epstein and. W. E. Lange 

157. Detection and determination 

of glue in rubber goods. 
India Rubber World 61, 216- 
7 (1920). 
S. W. Epstein and R. L. Moore. 

158. Determination of cellulose in 

rubber goods. II. S. Bureau 
of Standards Tech. Paper 
154; Rubber Age 6, 289-93 
(1920). 
W. Esch 

159. Determination of sulfur in 

rubber. Chem. Ztg. 2S, 200 
(1904). 

160. Contributions to rubber in- 

vestigations. Gummi Ztg. 
22, 766. 

161. Application of the bromide 

derivative methods, for the 
determination of vulcanized 
rubber goods. Chem. Ztg. 
35, 971-2 (1911). 
W. W . Evans and Ruth Merling 

162. A rapid bomb method for de- 

termination of sulfur in rub- 
ber compounds. Rochester 
meeting of the American 
Chemical Society, April, 
1921. 
G. Fendler 

163. New method for the analysis 

of rubber. Ber. Pharm. Ges. 
14, 208-15 (1904). 

164. Determination of rubber as 

tetrabromide. Gummi Ztg. 
24, 782 (1910). 

165. Determination of rubber as 

nitrosite. Gummi Ztg. 24, 
1000 (1910). 
G. Fendler and O. Kuhn 

166. Studies on rubber and rubber 

analysis. Gummi Ztg. 22, 
132-4; 160-3; 215-9; 249-52 
(1907). 

167. The determination of rubber 

as the tetrabromide. Gummi 
Ztg. 22, 710 (1908), 



BIBLIOGRAPHY 



127 



Harry L. Fisher and Harold Gray 

168. The tetrahydroxyphenol de- 

rivative, and its tetramethyl 
ether. New York meeting of 
the American Chemical So- 
ciety, Sept., 1921. 
Harry L. Fisher, Harold Gray and 
Ruth Merling. 

169. A discussion of the tetrabro- 

mide method for determin- 
ing rubber hydrocarbons. J. 
Ind. Eng. Chem. 13, 1031-4 
(1921). 
C. P. Fox 

170. Technical determination of 

rubber in Guayule. J. Ind. 
Eng. Chem. 1, 735 (1908). 

171. Effect of copper on crude rub- 

ber. J. Ind. Eng. Chem. 9, 
1092-3 (1917). 
F. Frank 

172. Determination of antimony in 

red rubber goods. Gummi 
Ztg. 25, 52, 2002 (1911). 
Frank and Birken 

173. Determination of mercuric sul- 

fide and antimony trisulfide 
in rubber goods. Chem. Ztg. 
84, 49. 
Frank and Jacobsohn 

174. Determination of cinnabar 

and antimony sulfide in red 
vulcanized rubber. Gummi 
Ztg. 23, 1046. 
F. Frank and E. Marckwald 

175. The analysis of rubber goods. 

Chem. Ztg. 26, 385 (1902). 

176. On the determination of total 

sulfur. Gummi Ztg. 17, 71 
(1903). 

177. Method for the direct deter- 

mination of the mineral mat- 
ter in rubber goods. Gummi 
Ztg. 22, 1344 (1908). 

178. Control of the analysis of rub- 

ber goods. Gummi Ztg. 23, 
1522 (1909). 

179. The chloral hydrate method 

in rubber analysis. Gummi 
Ztg. 23, 979 (1909). 

180. Direct determination of nitrog- 

enous substances and the 
impurities in raw rubber. 
Gummi Ztg. 26, 936 
(1912). 

181. Action of copper and other 

metals on rubber. I. De- 
terioration of cable insula- 
tion due to insufficient 



vulcanization. Gummi Ztg. 
28, 1280-3 (1914). 

182. Analytical methods for sulfur 

chloride. Gummi Ztg. 28, 
1580-1 (1914). 

183. The "Dracorubin" test for sol- 

vents used in the rubber in- 
dustry. Gummi Ztg. 80, 
524-5 (1916). 
Frank, Marckwald and Leibschutz 

184. On deresinated rubber. Gum- 

mi Ztg. 21, 366 (1907). 
H. C. T. Gardener 

185. Rubber. Chemist Druggist 79, 

3867. 
Gasparini 

186. Total sulfur by electrolytic 

method. Chim. Ital. 87, II; 
426-61 (1907). 
R. Gaunt 

187. The estimation of sulfur in 

rubber. Analyst Jfi, 9-10 
(1915). 
W. C. Geer 

188. Accelerated life of rubber com- 

pounds. India Rubber 
World 55, 127-30 (1916). 
W. C. Geer and W. W. Evans 

189. Ten years' experience with 

aging tests. India Rubber 
World 64, 887-92 (1921). 
P. Goldberg 

190. Determination of rubber in 

India rubber goods. Chem. 
Ztg. 37, 85. 
A*. Gorier 

191. Tackiness of crude rubber. 

Gummi Ztg. 80, 351 (1916). 

192. The viscosity index as a stand- 

ard for the preliminary test- 
ing of the quality of rubber. 
Chem. Zentr. I. 393-4 
(1916). 
K. Gottlob 

193. Influence of nitrous acid on 

rubber. Z. angew. Chem. 
20, 2213-21 (1907). 

194. Nitrosites of rubber and their 

use in analysis. Gummi Ztg. 
26, 1561. 

195. Vulcanization accelerators 

Gummi Ztg. 80, 303-8; 326- 
36 (1916). 
Frank Gottsch 

196. Specification of vulcanized rub- 

ber by volume, and its de- 
termination by a new solu- 
tion method. J. Ind. Eng. 
Chem. 7, 582-6 (1915), 



128 



THE ANALYSIS OF RUBBER 



J. Gram 

197. Technical rubber analysis. 

Chem. Ztg. 36, 249. 
Henry Green 

198. Volume increase of com- 

pounded rubber under strain. 
J. Ind. Eng. Chem. 13, 1029- 
31 (1921). 

199. Recent development in the art 

of rubber microsectioning. 
J. Ind. Eng. Chem. 13, 1130- 
2 (1921). 
■I. M. Grove 

200. The relative accelerating ac- 

tion of different compounds 
of lead in the vulcanizing 
of rubber. India Rubber 
World 64, 633-4 (1921). 
Alice Hamilton 

201. Industrial poisons used in the 

rubber industry. U. S. Bu- 
reau of Labor Statistics No. 
179 (1915). 
C. Harries 

202. On the reaction of rubber 

with nitric acid. Ber. 34, 
2991-2 (1901). 

203. The chemistrv of Para rubber. 

Ber. 35, 3256; 4429 (1902). 

204. The determination of rubber 

(nitrosite). Ber. 36, 1937 
(1903). 

205. Remarks concerning Weber's 

"dinitro-rubber." Ber. 38, 
87-90 (1904). 

206. Regarding the action of N-0 2 

on rubber. Z. Angew. Chem. 
20, 1969 (1907). 

207. Identification of the eight 

carbon-ring in normal rub- 
ber. Ber. 46, 2590 (1912). 

208. Detection of synthetic rub- 

ber in technical analysis. 
Gummi Ztg. 33, 222-3 
(1920). 
C. Harries and H. Rim-pel 

209. Determination of rubber as 

tetrabromide in crude rub- 
ber. Gummi Ztg. 23, 1370 
(1909). 
E. Hatschek 

210. The stress-elongation curve of 

vulcanized India rubber. J. 
Soc. Chem. Ind. Ifi, 251-3T 
(1921). 
L. G. D. Healy 

211. An electric desiccator for the 

analysis of India rubber and 
other organic compounds. J. 



Ind. Eng. Chem. 5, 489-90 
(1913). 
A. Helbronner and G. Bernstein 

212. Rubber solutions vulcanized by 

ultra-violet rays. Rubber 
Ind., 156-63 (1914). 
T r . Henri 

213. Action of ultra-violet rays on 

rubber. Caoutchouc & Gut- 
tapercha 7, 4371 (1910). 
R. Henriques 

214. Contributions to the analysis 

of rubber goods. Chem. Ztg. 
16, 1595; 1623; 1644 (1892); 
11, 634-8; 707-9 (1893); 18, 
411-2; 442-4; 905 (1894); 19, 
235; 382 (1895). 

215. Valuation of crude and manu- 

factured rubber. J. Soc. 
Chem. Ind. 16, 566-7 
(1897). 

216. Analytical examination of rub- 

ber goods. Z. angew. Chem. 
34, 802 (1899). 

217. Analysis of India rubber. 

Gummi Ztg. 14, 149; 165; 
183; 197 (1899). 
G. H. Hillen 

218. About rubber and guttapercha. 

Arch. Pharm. 251, 94-121 
(1913). 

219. The determination of cellulose 

in rubber. Gummi Ztg. 30, 
670-1 (1916). 
F. W. Hinrichsen 

220. Communications upon the 

Chemistry of rubber I; (a) 
with W. Manasse: On the 
tetrabromide of rubber; (b) 
with W. Manasse: Deter- 
mination of mineral fillers; 
(c) with W. Manasse: The 
determination of total sul- 
fur; (d) with K. Meisen- 
burg : Cold vulcanization ; 
(e) with E. Stern: hot vul- 
canization. Chem. Ztg. 3d, 
735-56 (1909). 

221. Analysis of vulcanized rubber 

goods ; Determination of 
fillers. Chem. Ztg. 33, 813, 
979, 1061 (1909). 

222. Control of rubber material for 

insulated conductors. Chem. 
Ztg. 34, 184 (1910). 

223. On the chemistry of rub- 

ber. II. The tetrabromide 
method. Gummi Ztg. 26, 
1408 (1912). 



BIBLIOGRAPHY 



129 



224. Present state of rubber analy- 

sis. Gummi Markt. 6, 253-7. 

225. The theory of the vulcaniza- 

tion of rubber. Mitt. kgl. 
Materialprufungsamt 33, 407- 
15 (1915). 
Hinrichsen and Kemp} 

226. The action of iodine on rub- 

ber. Ber. 46, 1287-91 (1913). 
Hinrichsen and Kindscher 

227. Identification of Para and 

Cevlon rubber. Chem. Ztg. 
34/230 (1910). 

228. Direct determination of rubber 

as tetrabromide. Chem. Ztg. 
35, 329-30 (1911). 

229. Hubener's method for the de- 

termination of rubber as 
bromide. Chem. Ztg. 36, 
217; 236 (1912). 

230. The desulfurization of vulcan- 

ized rubber. Koll. Z. 10, 
146-8 (1912); 11, 38-9 (1913). 

231. The theory of the vulcaniza- 

tion of rubber. Koll. Z. 11, 
191 (1913). 

232. Direct determination of rub- 

ber as tetrabromide. Z. anorg. 
Chem. 81, 70-82 (1913). 

233. The action of sulfur chloride 

and sulfur on rubber. Ber. 
46, 1291 (1913). 
Hinrichsen and Marcusson 

234. Rubber resins. Z. angew. 

Chem. 23, 49 (1910). 
Hinrichsen, Quensell and Kindscher 

235. Addition compounds of hydro- 

gen ha 1 ides and halogens 
with rubber. Ber. 46, 1283- 
7 (1913). 
If. II. Hodgson 

236. Notes on a recent comparison 

study of methods for deter- 
mining sulfur in rubber. 
India Rubber J. 47, 315 
(1914). 
I). Hold* 

237. Analysis of India rubber 

goods. Mitt. kgl. tech. 
Versuchsamt. 1892; p. 366; 
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Matthew Howie 

238. Determination of nitrogen 

content of rubber. J. Soc. 
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239. Analyses and analytical 

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240. 
241. 

242. 
243. 

244. 

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246. 
247. 

248. 

249. 
250. 



rubber. Chem. Ztg. 33, 144; 
155 (1909). 
Method for determining rub- 
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goods. Chem. Ztg. 
662 (1909). 
method for estimat- 
sulfur of vulcaniza- 

hard 
Gummi 



rubber 
S3, 648, 
A simple 
ing the 
tion in 
rubber. 



vulcanized 



Ztg. 



24, 



213 (1910). 
Concerning hard vulcanized 

rubber. Gummi Ztg. 24; 627- 

9; 740 (1910). 
Determination of rubber in 

crude rubber as tetrabro- 

740 



Ztg. 



24, 



mide. Gummi 
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Direct determination of rubber 
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Chem. Ztg. 35, 113-5 (1911). 

Direct determination of rub- 
ber in vulcanized rubber 
goods. Chem. Ztg. 34, 1307- 
8; 1315-6 (1910). 

Rubber tetrabromide. Gummi 
Ztg. 25, 634; 751-2 (1911). 

The determination of total sul- 
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(1912). 

The bromide method for the 
determination of rubber. 
Gummi Ztg. 26, 1281 (1912). 
Cf. also Gummi Ztg. 28, 320 
(1914). 

On the oxidation of rubber. 
Gummi Ztg. 28, 237-9 (1913). 

The determination of syn- 
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88, 361-2 (1919). 

ut in 

Drying the acetone extract of 
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B0, 212-3 (1915). 

252. Determination of total sulfur 

in the products of the rub- 
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anal. 20, 214-6 (1915). 

253. Observations on the subject of 

the determination of total 
chlorine in rubber substi- 
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241-2 (1915). 

254. Rapid analysis of commercial 

sulfides of antimony. Ann. 
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255. Vulcanization accelerators. 

of vulcanization. Mon. Sci. 
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A. H 
251. 



130 



THE ANALYSIS OF RUBBER 



256. Vulcanization accelerators. 

Caoutchouc & Guttapercha 
15, 9596-8 (1918). « 
H. Ingle 

257. Some observations on rubber 

resins. J. Soc. Chem. Ind. 
SI, 272-3 (1912). 
International Rubber Testing Com- 
mittee 

258. Suggestions for the standard 

testing of rubber. Gummi 
Ztg. 25, 1277 (1911). 
Felix Jacobson 

259. The determination of fillers. 

Gummi Ztg. 27, 1906 (1913). 
J. Jacoby 

260. A new extraction apparatus. 

Gummi Ztg. 27, 1870 (1913). 
Joint Rubber Insulation Committee 

261. See W. A. Del Mar 
W. Jones 

262. Analysis of rubber compounds. 

Elec. World 59, 320. 
H. W . Jones 

263. The use of nitric acid as a sol- 

vent for compounded and 
vulcanized rubber. The 
Rubber Ind. (1914), p. 189. 

264. Methods of determining small 

amounts of carbon dioxide 
in rubber goods in the pres- 
ence of sulfides. The Rub- 
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Chem. Ind. 84, 672 (1915). 

265. A simple method for the de- 

termination of mineral mat- 
ter in rubber mixings. The 
Rubber Ind. 1914, 199-200. 
A. W. Jones, Jr. 

266. Estimation of mineral matter 

in vulcanized rubber. Chem. 
Analyst 20, 21 (1917). 
Kaye and Sharp 

267. Rapid method for the estima- 

tion of sulfur in vulcanized 
rubber. India Rubber J. 44, 
1189 (1912). 
W. J. Kelly 

268. The determination of the true 

free sulfur, and the true co- 
efficient of vulcanization in 
vulcanized rubber. J. Ind. 
Eng. Chem. 12, 875-8 
(1920). 

269. Determination of true free 

sulfur, and the coefficient of 
vulcanization in vulcanized 
rubber. II. J. Ind. Eng. 
Chem. 14, 196-7 (1922). 



Andrew H. King 

270. The chemical analysis of rub- 

ber goods. Met. Chem. Eng. 
14,581-4 (1916). 

271. Rubber vulcanization accel- 

erators. Met. Chem. Eng. 
15, 231-4 (1916). 

272. Rubber substitutes. Mot. 

Chem. Eng. 18, 630-6 
(1918). 
F. Kirchhof 

273. The direct determination of 

rubber by titration with bro- 
mine. Gummi Ztg. 27, 9 
(1913). 

274. On the oxidation of rubber. 

Roll. Z. 13, 49-61 (1913). 

275. The regeneration of rubber 

from its tetrabromide. Koll. 
Z. 15, 126-31 (1914). 

276. The action of concentrated 

sulfuric acid on natural and 
artificial rubber. Koll. Z. 
17,311-5 (1920). 
Martin Klassert 

277. Rubber resins. Z. angew. 

Chem. 26, 471-2 (1913). 
0. H. Klein, J. H. Link and F. 
Gottsch 

278. The determination of mineral 

fillers in rubber by the ani- 
line method. J. Ind. Eng. 
Chem. 9, 140-1 (1917). 
/. L. Kondakov 

279. Syntheses of rubber. Caout- 

chouc & Guttapercha 18, 
10980-84; 11097-100 (1921); 
19, 11169-71 (1922). 

F. von Konek 

280. Rapid estimation of sulfur in 

organic compounds, coals, 

mineral oils, etc. Z. angew. 

Chem. 22, 516 (1903). 
L. dr Konivgh 
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in rubber goods. J. Am. 

Chem. Soc. 19, 952-6 (1897). 
Otto Korneck 

282. Critical investigation of the 

methods of analysis of crude 
rubber. Gummi Ztg. 25, 
4-9; 42-6; 77-88 (1910). 

283. Determination of rubber as 

tetrabromide. Gummi Ztg. 
25, 424 (1911). 

G. D. Kratz 

284. Rate of vulcanization and 

factory output. India Rub- 
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G. D. Kralz and Arthur H. Flower 

285. The vulcanization of rubber at 

constant temperatures and 
by a series of increasing tem- 
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Chem. 11, 30-3 (1919). 

286. Effect of certain accelerators 

upon the properties of vul- 
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287. The effect of certain accelera- 

tors on the properties of vul- 
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G. D. Kratz, A. H. Flower, and Cole 
Coolidge 

288. The action of certain organic 

accelerators in the vulcan- 
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289. The action of certain organic 

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H. Kuhl 

290. Determination of lead and 

zinc in rubber goods. Apoth. 
Ztg. 51, 135. 
A. A. Ladon 

291. Testing rubber insulation. 

Met. Chem. Eng. 14, 560 
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Alan Leighton 

292. The inflammability of gas 

black. India Rubber World 
60, 425-6 (1919). 
M. Levin and S. Collier 

293. An improved method for the 

determination of sulfur in 
rubber. Rubber Age 9, 47-8 
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W. K. Lewis and W. H. McAdams 

294. A direct method for the 

determination of rubber hy- 
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vulcanized rubber. J. Ind. 
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Lindley 

295. Some types of mineral rubber. 

India Rubber World 47, 17. 
J. H. Link 

296. Preparation of rubber samples. 

Chem. Analyst 15, 7 (1915). 
II. Loewen 

297. Determination of fillers. 

Gummi Ztg. 28, 7 (1914). 



C. A. Lobry de Bruyn and F. II. 
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298. Analysis of India rubber 

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B. D. W. Luff and B. D. Porritt 

299. The determination of avail- 

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W. C. McLewis 

300. The properties of the colloidal 

state and their applications 
to industry. Caoutchouc & 
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(1920). 
Marckwald and Frank 

301. Magnesia in rubber com- 

pounds and vulcanization. 
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302. The determination of the sul- 

fur and halogen content of 
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130. 
Marcusson and Hinrichsen 

303. The determination of fillers in 

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Marquis and Heim 

304. A method for determining 

rubber in crude rubber. 
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A. Maximov 

305. The acceleration of vulcaniza- 

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Caoutchouc & Guttapercha 
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306. Analysis of vulcanized rubber. 

Caoutchouc & Guttapercha 
17, 10233 (1920). 
A. M. Munro 

307. A new rapid method for the 

determination of sulfur in 
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Rubber World 62, 426 (1920). 
R. Nicolardot 

308. Examination of a sample of 

synthetic rubber. Ann. 
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176-9. 

C. Olin North 

309. The effect of compounding in- 

gredients on the physical 
properties of rubber. India 
Rubber World 63, 98-102 
(1920). 



132 



THE ANALYSIS OF RUBBER 



310. Mineral rubber. India Rub- 

ber World 65, 191-2 (1921). 
G. Noycr 

311. Analysis of rubber overshoes. 

Caoutchouc & Guttapercha 
14, 9272-4 (1917). 

312. Flowers of sulfur and sub- 

limed sulfur. Caoutchouc & 
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/. /. Ostromuislenskii 

313. A new method of solid vulcan- 

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314. Mechanism of the action of 

amines and metallic oxides 
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315. A new method of vulcanizing 

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316. The preparation of vulcanized 

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317. A new method of vulcanizing 

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318. Vulcanization or synthetic 

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319. Mechanism of the process of 

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320. Hot vulcanization of rubber 

by nitro compounds without 
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321. Hot vulcanization of rubber 

by peroxides or peracids, 
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Phys. Chem. Soc. 47, 1467- 
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322. The action of gaseous oxygen 

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323. Vulcanization accelerators. 

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324. The oxidation of rubber. J. 

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325. A new process for the vulcan- 

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326. Determination of fillers in rub- 

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0. Pfeiffer 

327. Determination of sulfur by 

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H. Pohle 

328. Contributions to the knowl- 

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M . Pontio 

329. A new method for determin- 

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(1908). 

330. Determination of organic and 

mineral impurities in crude 
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6, 2752 (1909). 

331. Determination of total sulfur 

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332. Commercial analysis of manu- 

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333. A direct method for determin- 

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334. Direct estimation of pure rub- 

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BIBLIOGRAPHY 



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363. Volume increase of com- 

pounded rubber under strain. 



134 



THE ANALYSIS OF RUBBER 



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369. The isolation of the in- 

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370. Bromination and constitution 

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371. Rapid determination of golden 

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372. Valuation of India rubber. 

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136 



THE ANALYSIS OF RUBBER 



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13S 



THE ANALYSIS OF RUBBER 



Lothar E. Weber 

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Ernst Wleck 

494. Contribution to the investiga- 

tion of golden sulfide ; detec- 
tion and determination of 
calcium sulfate. Gummi Ztg. 
29, 479-80 (1915). 
Van Lear Woodward 

495. Antimony sulfide; its place in 

tube making. Rubber Age 
1, 99-100 (1917). 
Fred E. Wright 

496. Oblique illumination in petro- 

graphic microscope work. 
Am. J. Sci. 35, 63-82. 

497. The measurement of the re- 

fractive index of a drop of 
liquid. J. Wash. Acad. 4, 
269-79; C. A. 8, 2516. 

498. Determination of the relative 

refringence of mineral grains 
under the petrographic mi- 
croscope. J. Wash. Acad. 4< 
389-92; 5, 101-7. 

499. Recent improvements in the 

petrographic microscope. J. 
Wash. Acad. 6, 465-7 (1916) ; 
C. A. 10, 2419. 

500. Petrographic microscope in 

analysis. J. Am. Chem. Soc. 
38, 1647-58 (1916). 

501. Polarized light in the study of 

ores and metals. Proc. Am. 
Phil. Soc. 58, 401-47 (1919) ; 
C. A. 14, 1645. 



APPENDIX A. 
The Preparation of Materials for Rubber Manufacture. 

While to many readers, the factory processes concerned with the prep- 
aration of the materials which enter into the composition of a rubber 
compound are every day affairs, for the benefit of those who are not 
familiar with the technical processes used in rubber factories, it seems 
worth while to give some brief description of the various steps. The crude 
rubber is prepared by washing, if necessary, and by "milling" or "breaking 
down"; the pigments may be screened or bolted. Certain pigments and 
organic accelerators are incorporated with rubber to form master batches. 
These, together with such pigments and crude rubbers as are used in the 
form in which they are received, are fed to the compound room, where 
the batches are weighed out according to the prescribed formulas. These 
batches are mixed in the mill room, and then stored to cool and age. 
The mixed rubber compound is prepared for manufacturing purposes by 
(a) calendaring into sheets or strips; (b) tubing into special or irregular 
shapes; (c) made into a rubber cement by dissolving the rubber com- 
pound in benzene, gasoline, or a mixture of the two; (d) applied to fab- 
ric, either on a calendar, or on a spreading machine. These four classes 
may be called the "intermediates" of the rubber industry; from these, 
practically all rubber articles are built. 

Washing. The wild rubbers, and the inferior grades of plantation rubber, 
contain large amounts of bark, dirt, and other foreign matter, and in this 
condition are not suitable for rubber manufacture. These impurities are 
removed by washing. The better grades of plantation rubber may be 
washed in order to be certain that no grit or dirt remains in the rubber, 
to cause trouble during the service of the articles of which they may form 
a part. 

The washing equipment consists of a vat for heating the rubber, and 
three washing mills — a cracker, a refiner, and a finisher. Some factories 
combine the refining and finishing on one mill. 

Some of the wild rubbers, notably the fine Para sorts, come in large 
lumps, or biscuits. Before they are ready for washing, they must be boiled 
in water for several hours in order to soften them up sufficiently to be 
worked easily. The large lumps are cut up before being heated. 

The cracker consists of two rolls with heavy, coarse corrugations. The 
rubber is passed through this mill two or three times, just enough to form 
a thick, rough sheet. 

The refiner is a two-roll mill, the front roll having finer corrugations 
than the cracker. A stream of water plays on the rubber as it passes 
between the rolls, and as fresh surfaces of the rubber are exposed, the 
water washes out the impurities. 

The finisher is a mill similar to the refiner, but having even smaller 
corrugations on the front roll. Here the rubber is sheeted out thin, ready 
for drying. 

Drying. Two processes are used for drying: "air-drying" and "vacuum 
drying." 

In the air-drying, the lone sheets from the washing mills are hung over 
poles in a drying room, the temperature of which is maintained at 90-110F. 

139 



140 THE ANALYSIS OF RUBBER 

Fresh air is kept circulating through the room, so as to facilitate drying. 
In the plantation rubbers, the moisture is usually on the surface only, and 
such rubbers can be dried in from two to four days. Many of the wild 
rubbers contain a large amount of moisture, and take up a further quan- 
tity during the boiling and washing. This moisture is distributed through- 
out the rubber, and drying takes from three to five weeks. 

In vacuum drying, the rubber is laid on pans, which are placed in a 
steam-heated vacuum oven. The process requires only a few hours, but 
it is not particularly well adapted for drying the wild rubbers, since the 
rapid evaporation tends to form a hard surface film, which hinders further 
evaporation, prolonging the drying to such a degree as to materially soften 
and injure the rubber. Soft wild rubbers, such as Guayule, Pontaniak, 
and the Africans, flow so readily at ordinary temperatures that they can- 
not be dried on poles, and must be dried in pans, either in the drying 
rooms or in the vacuum dryers. 

Milling, or Breaking Down. Crude rubber varies considerably in quality 
and in order to lessen the variation, lots of rubber are averaged by the 
process of "milling" or "breaking down." For this purpose, we use the 
regular mixing mill, consisting of two smooth rolls, the rear one having 
a higher circumferential speed than the front one. The rolls are hollow, 
so as to permit them to be heated, or cooled, in order that they may be 
maintained at a temperature of about 175F. After the mills have been 
running a short time, the friction of the rubber is more than sufficient to 
maintain this temperature, and cold water is kept running through them 
to prevent them getting so hot as to injure the rubber. 

The rubber is fed into the mill, and as it softens it adheres to the front 
roll. It is cut off, rolled up, and fed back into the mill until it is 
homogeneous. By this time, it has attained a soft, wax-like consistency. 
When sufficient uniformity has been attained, the rubber is cut off in the 
form of sheets, or rolls. 

This "milling" not only produces a uniform grade of rubber, but it 
breaks up the hard, tough particles of crude rubber, and prepares it for the 
following operations of mixing, calendaring, and tubing. 

Master Batches. Some pigments, and many of the organic accelerators, 
are first mixed with rubber, in what are known as "master batches," before 
they are sent to the compounding room (where the batches for making 
the various rubber compounds are weighed out). Pigments such as glue, 
which requires a longer mixing than can safely be given the entire com- 
pound, are first mixed with rubber. Glue, for example, is usually mixed 
in the proportion of two or three parts of rubber to one of glue. Dusty 
pigments, such as gas black, are treated in a similar way, frequently in a 
building separated from the regular mixing room. This step is necessary 
to prevent the black dust settling down on the other mills, and injuring 
the colors of the other batches. Organic accelerators are used in small 
amounts, sometimes 0.2% or less. It is highly important that these small 
amounts be very evenly distributed throughout the entire compound. 
After these accelerators have been thoroughly incorporated into a master 
batch, it is very simple to distribute them throughout a compound. A 
further advantage is found in the improved accuracy of weighing out the 
small amounts needed, and elimination of losses of the accelerator during 
the mixing. Small errors or losses in getting the accelerator into a com- 
pound will produce more variation in the finished product than in any 
other single ingredient. 

Screening, or Boiling. Foreign matter, such as wood. grit, pieces of 
paper or twine from the containers in which the pigments are shipped, are 
objectionable in many of the better grades of rubber articles. Such 
impurities are removed from the pigments by sifting the latter through 
screens of from 40 to 90 mesh, the size depending largely upon the pig- 
ment, and the article in which it is to be used. 



APPENDIX 141 

Compounding. The compounding consists simply in weighing out the 
proper amounts of pigments and rubbers. These are placed in large iron 
boxes or pans, and the total weight checked up before leaving the com- 
pound room. The usual tolerance in weighing is 0.5 lb. over or under, in 
a 100 lb. batch, or a total variation of 1%. 

Mixing. The mixing mills used for mixing rubber compounds vary from 
36 to 84 inches in length, and 20 to 24 inches in diameter. The amount 
of stock that can be mixed on a mill is largely a function of the width 
and diameter of the front roll of the mill, and the specific gravity of the 
stocks; these batches will range from 25 to 225 lbs. 

The rubber is first thrown on the mill, together with any pigments put 
up in master batches, 1 reclaimed rubber, and mineral rubber. After the 
rubber softens, the pigments and oils are added. Any material which 
drops between the rolls is caught in a pan, and returned to the rolls until 
everything has been incorporated. The mixing is continued until the com- 
pound is homogeneous, after which it is cut off from the rolls in slabs of 
about one quarter to three-eighths of an inch in thickness, and weighing 
15 to 25 lbs. each. These slabs are laid on racks to cool, after which they 
are sent to storage bins to age for 24 to 48 hours. 

Calendaring. A calendar consists of three smooth hollow steel rolls, 
accurately ground, arranged vertically, the top and bottom rolls revolving 
in the opposite direction to the middle roll. The stock, which has been 
previously softened or "warmed-up" on the regular mixing mill, is fed 
into the calendar between the top and middle rolls, passes around the 
middle and under the bottom roll, after which it is wrapped up between 
cotton liners. The function of these liners is to keep the fresh surfaces 
of the rubber apart, and protect them from dust and dirt. The thickness 
of the sheet is controlled by the distance between the top and middle 
rolls; the width by adjustable knives placed against the rear of the middle 
rolls. The middle and bottom rolls are further apart than the thickness 
of the rubber sheet. The bottom roll is cooled, which serves to cool 
the rubber, and toughen the sheet, before it is wound up in the liner. 

The rubber sheet may be made of the desired thickness in one passing 
through the calendar, or it may be built up from a number of thinner 
sheets. 

Calendaring Rubber to Fabric. When a rubber compound is to be 
applied to a roll of fabric, the rubber is fed into the calendar as above, and 
the fabric is fed from the opposite side, between the middle and bottom 
rolls. The first coat is put on by having the middle roll rotate at a faster 
speed than the bottom one. giving a grinding action which forces the 
rubber into the meshes of the fabric. This is called "frictioning." If a 
further layer of rubber is desired over the friction coat, the rubber and 
fabric are fed in as before, but the middle and bottom rolls rotate at the 
same speed, and the sheet of rubber is thus laid on the fabric, and pressed 

1 We have here an added advantage in using organic accelerators in the form 
of master batches. The accelerator is uniformly distributed throughout 
the rubber before any of the sulfur is added, thus eliminating largely the 
danger of partial vulcanization on the mixing mill (called burning, or scorch- 
ing). It is a well established fact that high concentrations of many of the 
organic accelerators greatly increase the reaction of vulcanization, especially 
at the lower temperatures existing on the mixing mill. A compound containing 
0.50% of dimethyldithiocarbnmato burned in less than one minute after the 
sulfur has been added, whereas a similar stock, having only 0.05%, mixed with- 
out difficulty. Andres (Caoutchouc & Guttapercha 18, 11089-97 [1921]), showed 
that 2.5% of thiocarbanilide gave the best cure at 50 minutes, whereas 5% 
gave a good cure in 3 minutes, and the best cure in 5 minutes. These higher 
concentrations are easily obtained in a poorly mixed stock, and obviously in- 
crease the probability of damage to the stock. 



142 THE ANALYSIS OF RUBBER 

together just sufficiently for them to adhere firmly. This coating of rubber 
is called the "skim" coat; tire fabric, for example, is usually frictioned on 
both sides, and skim-coated on one side only. After the skim has been 
applied the fabric is rolled up between a liner, to be cut up later as desired. 

Tubing. The tubing machine is used for irregular strips of rubber, such 
as tire treads, tire beads, rubber tubing, and the insulation on wire. The 
essential parts of a tubing machine are a hopper for feeding in the stock, 
the barrel, which can be heated by steam, and which contains the screw 
for carrying the stock to the head, and the head, containing the die through 
which the stock is forced. The head is often heated with a gas jet, to 
prevent the tubed rubber cooling in the die and coming through rough or 
cracked. The rubber is warmed up to the desired consistency, cut into 
small strips, and fed into the hopper. The stock as it comes from the 
tubing machine is cut in the desired lengths, placed between liners, and 
set aside to cool. 

Cement. In the manufacture of cement, the rubber is warmed up on 
the mixing mill, and cut off in very thin sheets. A weighed amount of 
this rubber is cut into small pieces, and thrown into a churn or mixer, 
to which has already been added the measured amount of solvent. The 
contents of the mixer are stirred until solution is complete, a matter of 
from 4 to 12 hours, depending largely upon the nature of the 
solvent, the grade of rubber, and the efficiency of the mixer. The first 
mixing usually gives a heavier cement than desired, and this is thinned 
down with more solvent until the right viscosity is obtained. The nature 
of the service for which the cement is intended dictates the degree of 
viscosity. 

Spreading. In addition to the method of applying rubber to fabric by 
calendaring, as described above, we may use the process known as "spread- 
ing." A spreader consists of a rubber coated roll, against which rests a 
heavy knife. Beyond the rubber roll are steam-heated coils or plates, 
about 18 to 30 feet in length. The rubber compound is first made into 
a very heavy cement (generally called "dough"). The fabric is passed 
between the rubber-coated roll and the knife, the dough is applied to the 
fabric just before the latter reaches the knife, and in passing between the 
roll and the knife the latter scrapes off all but a thin coating of the cement. 
As the fabric passes over the heated plates, the solvent in the cement 
evaporates, and leaves a thin coating of rubber in very intimate contact 
with the fabric. The space between the knife and roll controls the amount 
of rubber left upon the fabric. The amount of rubber which may be added 
at one passing depends upon the ability of the spreader to drive off the 
solvent during the time when the fabric is passing over the heated rolls. 
The factors are the temperature and length of the drying plates, and the 
speed at which the machine is driven. A heavy coating of rubber is ob- 
tained by passing the fabric through until the desired quantity of rubber 
has been applied. 



APPENDIX B. 
Physical Tests. 

The chemist in the rubber factory is usually given the duty of making 
whatever physical tests may be necessary to determine the properties of 
the rubber compound or finished article. Similarly, the chemist in the 
consumer's laboratory supervises and interprets the results of the physical 
tests made upon samples taken from deliveries of manufactured goods. 
It seems desirable, therefore, to point out what physical tests are usually 
made, and their relation to the quality and life of the material. 

The principal physical tests are (1) tensile strength, (2) ultimate elonga- 
tion, (3) set at break, (4) friction. These tests are usually made on the 
same testing machine. The tensile st length is the force required to break 
a unit area of a rubber compound: the ultimate elongation is the extcn! 
to which the rubber can be stretched before it will break; the set at 
break is the increase in length of a measured length of rubber, taken at 
some definite time after break; and the friction is the force required to 
separate a rubber compound from a piece of fabric to which it has been 
vulcanized. 

Tensile Testing Machine. Three types of machines arc in more or less 
common use in this country: (a) Scott; (b) Bureau of Standards; (c) 
Schopper. 

(a) The Scott is a machine of the dead weight type, the pull being 
against a lever which moves outward as the tension is applied. There 
are two clamps, into which are inserted the ends of the rubber test pieces. 
The upper clamp is attached to the end of the weighted lever; the lower 
clamp is attached to a rod driven at a uniform rate of speed (usually 20 
inches per minute for tensile tests, and 2 inches per minute for friction 
tests). The lever carries a set of pawls, which engage in the teeth of a 
curved rack, preventing the lever from falling back when the tension is 
released (as for example, when the test piece breaks). The tension is read 
off from a dial, the indicator being actuated by the motion of the lever. 

(6) The Bureau of Standards machine 1 differs from the Scott in that 
the pull is against a spring balance, which directly records the pull. The 
clamps are of the same type, and the operation of the machine is essen- 
tially the same as that of the Scott. 

(c) The Schopper machine is one of the dead weight type. The rack, 
over w r hich the lever moves, is graduated, and the tension is read off oppo- 
site the point where the lever stops. 

There are a number of styles of clamps which may be used with these 
machines, the principal ones being the eccentric grip, with its modification 
consisting of a number of thin disks, mounted eccentrically, the zig-zag 
grip, which is tightened by a screw, and the spool grips, for use with 
ring-shaped test pieces. Any of these types may be used with any of the 
machines mentioned, but the Scott and the Bureau of Standards machines 
usually carry the eccentric grip clamps, for testing bar-shaped test pieces, 
whereas the Schopper usually has only the spool grips. As long as the 
Schopper is equipped only with grips for testing ring-shaped test pieces, 

1 Cf. Bureau of Standards Circular 38, Fourth Edition, p. 53. 

X43 



144 THE ANALYSIS OF RUBBER 

it cannot be considered equivalent to the other machines. Between the 
Scott and the Bureau of Standards machines there is little choice to be 
made — providing they are both accurately calibrated, and the clamps sep- 
arated at the same rate of speed, comparable results may be obtained. 
The dead weight type is usually considered to be the more rugged, and 
less likely to get out of order, than the spring balance. 

When using the ring-shaped test pieces, any one of the three machines 
may be used without affecting the results. In fact, the type of machine 
is of importance, not so much for the accuracy of the determinations which 
it will give, but from the point of view as to how it will stand up under 
the service given to it, and the convenience of operation. 

Shape of Test Pieces. The test piece commonly used in this country for 
the determination of tensile strength, is the "bar-shaped" or "dumb-bell" 
test piece. The constricted part is either ~hk or %" and 1 or 2" long. The 
ends are enlarged to reduce to the minimum the danger of the test piece 
tearing in the clamps. The enlarged ends are 1" wide for the V±" width, 
and 1^4" wide for the %" width. A few use a V-k" width at the ends for 
a Vi" width at the constricted part, particularly for testing compounds of 
high rubber content (the so-called pure gum compounds). The style of 
test piece is largely a matter of the operator's choice, influenced in pait 
by the nature of the material to be tested. Some specifications define 
exactly the shape, leaving nothing to the discretion of the operator. While 
theoretically there should be no difference, as a matter of practice results 
are comparable only when the same shaped test piece is used. 

In the above, nothing has been said regarding the thickness of the test 
piece. Except when slabs are prepared particularly for the purpose of 
making tensile tests, this is not a matter which can be controlled easily, 
but the thickness is quite likely to be an important factor, the thicker 
pieces showing a greater tendency to tear, and hence giving lower results 
than would be obtained from thinner ones. The most satisfactory prac- 
tical range is from % to 3/16" (0.125 to 0.183"). 

The ring-shaped test piece cannot be compared with the b;u-.<haped test 
piece* Its only advantage lies in the fact that with it an autographic 
chart may be made of the stress-strain curve. With the bar-shaped test 
piece, to get the same data, it is necessary to use two operators, and from 
their observations plot the stress-strain curve. 

The' principal precautions to be taken in preparing test pieces of any 
shape are that the edges be cut evenly and that the opposite sides of 
the constricted part are parallel. With ring-shaped test pieces, the rings 
must be very accurately centered, so as to obtain the same cross-section 
at all points. If the top and bottom surfaces of the test pieces are not 
smooth, they should be made so by buffing, so that accurate readings of 
thickness may be made. 8 

Tensile Strength. The tensile strength is usually expressed in pounds 
per square inch, or kilograms per square centimeter. 4 The area is usually 
referred to the cross-section at rest. However, before rubber can be 
broken, it must be stretched from 300 to 900%, and since there is no 
change in volume " during the stretching, the cross-section at break is very 
much less than when the test piece is at rest. For this reason, the tensile 

•This point is argued very convincingly in the Bureau of Standards Circular 
38, Fourth Edition, p. 66, etc. 

•A buffing machine, suitable for the purpose, is described in Bureau of Stand- 
ards Circular No. 38, Fourth Edition, p. 48. 

*To convert lbs. / sq. in. into kg. / sq. cm., multiply by 0.07031; to convert 
kg. / sq. cm. to lbs. / sq. in., multiply by 14.222. 

• Schippel's change in volume on stretching refers only to the vacua formed 
around coarse particles of pigment. Such changes are negligible for the calcu- 
lations under discussion. 



APPENDIX 145 

strength has sometimes been referred to the cross-section at break, called 
the "tensile product." This figure is obtained by multiplying the tensile 
strength by the elongation at break. 

The tensile strength is appreciably affected by a considerable number 
of factors, some of which are within the control of the operator and some 
are not. Of these, the most important are: rate of separation of the jaws, 
temperature, size and shape of the test pieces, the direction of the cut 
(i.e., whether along the length of a calendared sheet, or across), previous 
stretching of the rubber, and the age of the rubber compound. These 
factors have been discussed at some length by Whitby, 8 and the Bureau 
of Standards, 7 and their conclusions may be briefly summarized as 
follows: 

Rate of Separation of the Jaws. The higher the speed, the higher will 
be the results for tensile strength and ultimate elongation. The range in 
speeds between 5" and 45" per minute may affect the results anywhere 
from 5% to 20%. The speed 'generally employed is 20" per minute. 

Temperature. The temperature at which tests are usually expected to 
be made is 70F. Increasing the temperature lowers the tensile strength 
and increases the elongation; lowering the temperature produces a reverse 
effect. It is worthy of notice that for the range of temperature from 50F 
to 90F. the tensile at break is much more constant than either the tensile 
strength or ultimate elongation. 

Size and Shape of the Test Pieces. There is a tendency for narrow test 
pieces to develop higher values than wider ones; between Yi" and %", 
differences as high as 20% have been noted. Unpublished data may con- 
tain instances of even greater variation. 

Direction of Cutting. The Bureau of Standards found that the tests 
made on samples cut in the direction of calendaring show a higher ten- 
sile and lower elongation than those cut in the transverse direction. 
Some experimenters have not been able to duplicate these results, but 
most of the data on the subject indicate that there is a decided difference 
between the two directions in a calendared sheet. The ring-shaped test 
pieces include rubber cut in all directions, and since the break occurs in 
the direction of least resistance, it is obvious that the effects of calendaring 
cannot be detected with such test pieces. 

Previous Stretching of the Rubber Test Pieces. It is curious that while 
Memmler and Schob, and the Bureau of Standards, agree that previous 
stretching alters the results of tensile tests, the former obtained lower 
results from test pieces subjected to previous stretching, whereas the latter 
obtained higher figures. Memmler and Schob tested ring-shaped test 
pieces by subjecting them to 50% of their normal breaking load for a 
period of 30 minutes, and testing them after a rest period of 24 hours. 
The Bureau of Standards employed bar-shaped test pieces, stretching to 
200%, releasing, and then continuing with an increase of 100% until failure 
ensued. With high grade material, Memmler and Schob found differences 
of about 35% loss in tensile strength ; the Bureau of Standards found in- 
creases of about 20%. Apparently what is needed to determine the exact 
difference caused by previous stretching, is to combine the two sets of 
ideas. The Bureau of Standards figures seem to show that short periods 
of stretching increase the tensile strength. By increasing the time of 
stressing the test pieces, we could find out whether or not there is a point 
at which there is no further increase in tensile properties. Similar experi- 
ments could be made with ring-shaped test pieces, for it is not at all im- 
possible that the differences may be largely attributed to the differences 
in the shape of the test pieces. 
The greatest importance of these experiments lies in the fact that they 

6 Whitby, Plantation rubber and the testing of rubber. 

7 Bureau of Standards Circular 38, Fourth Edition. 



146 THE ANALYSIS OF RUBBER 

emphasize the necessity for permitting rubber samples to age for at least 
48 hours, in order to be certain that they have reached equilibrium. 

Aging of the Rubber. Practically every one who has followed the test- 
ing of rubber is agreed that a certain time is required after vulcanization 
for the rubber to come to equilibrium. To be absolutely safe, many have 
placed the period for aging at 3 days; others think that as little as 24 
hours will suffice. In view of the results obtained in a study of the effects 
of previous stretching, 24 hours seems hardly enough to be on the safe 
side, and consequently a minimum of 48 hours is recommended. 11 

Ultimate Elongation. The ultimate elongation has been defined as being 
the extent to which a rubber compound may be extended before rupture 
will occur. With bar test pieces, the elongation is determined by placing; 
on the constricted portion of the test piece, parallel lines either 1" or 2" 
apart, and then stretching until it breaks. The distance between the 
marks at break (a) less the original distance (b) is. the elongation (c), 
and is expressed in percentage. Some prefer to express the ultimate 
elongation by dividing a by b, giving figures which are 100% higher than 
the more commonly accepted figures. There is nothing gained by this 
procedure, and it causes a great deal of confusion when making compari- 
sons. In order to avoid this, many specifications are now being written 
calling for an elongation in definite figures, such as from 1-5 inches, or 6 
inches, or whatever length may be desired. 

For correctly cured soft vulcanized rubber, the ultimate elongation is 
affected most by the amount and grade of rubber present. Compounds 
containing 90% of rubber will have an elongat'on of about 900% to 
1000%, while compounds containing only 30% will have an elongation of 
only 300% to 500%. Just as in the case of tensile strength, we find that the 
ultimate elongation is a more or less arbitrary figure, the value of which 
will depend to a considerable degree upon the manner of its determina- 
tion. Practically all of the factors which influence the values for tensile 
strength will be found to have an effect on those for elongation. 

Stress-Strain Curves. If the tensile strength for each increment of elonga- 
tion be determined, and plotted, the line drawn through these points 
gives us what is known as the "stress-strain" curve. Generally the strains 
are plotted as ordinates, and the stresses as abscissae. The Scott and 
Schopper machines plot these curves autographically when ring-shaped test 
pieces are used. With bar test pieces, one operator reads the strains, and 
the other the stresses. By plotting the curves of a series of cures on one 
sheet, the effect of time of vulcanization, or whatever other factor it is 
desired to follow, may be easily observed. 

The principal trouble with the stress-strain curve for rubber, is that in 
making the curves of a series of cures, the first half or three-fourths of 
the curves take practically the same course, and it is difficult, if not impos- 
sible, to notice any appreciable difference until the last quarter of the 
curve. The Goodyear laboratory has suggested a means for plotting the 
results in such a fashion as to bring out differences in the early parts 
of the curves. They plot stresses as ordinates, and the time of cure as 
abscissae; for each time of cure, they plot the tensile strength for 100% 
and each succeeding 100% elongation up to the break. Curves are drawn 
through all points having the same elongation, and the final curve is drawn 

• It is obvious that at times these precautions must yield to expediency, and 
it is frequently more important in manufacturing work to get immediate results 
which are approximately accurate, than to wait 48 or even 24 hours. We have 
frequently taken slabs of rubber out of a mold, cooled them in running water for 
15 minutes, and then proceeded with the tests. In all such cases, the proba- 
bility of large errors being present was known and appreciated. Those samples 
which showed any promise of being satisfactory were given the regular tests 
48 hours later, and the latter figures only were used for record and comparison. 



APPENDIX 147 

through the points of ultimate elongation. The latter is the usual "tensile- 
strength-time-of-cure" curve used so much by investigators in this coun- 
try. This system of plotting gives a much more satisfactory picture than 
does the ordinary stress-strain curve. 

Set at Break. After the test piece has been broken on the testing 
machine it is laid aside for a period which ranges from 1 to 24 hours, 
according to the methods adopted by the various laboratories, and the 
increase in the distance between the marks is measured, and calculated 
to percentage. The set at break for various cures of the same compound 
passes through a maximum at the optimum cure, the shape of the curve 
as plotted against time of cure very much resembles the tensile strength- 
time-of-cure curve. Very little practical use is made of this determination. 

A far more extensive use of the determination of set has been made 
by determining the set on test pieces which have been stretched to less 
than their ultimate elongation. The usual routine in such tests is to 
stretch the test piece for ten minutes" and measure the increase in elonga- 
tion ten minutes after releasing. With such tests, there is a drop in the 
value of the set from an undercure to an overcure, the effect being most 
noticeable in the former. 

Friction. The adhesion between fabric and rubber is termed "friction." 
There are two methods for its determination: (a) the amount of separa- 
tion under definite load; (b) the load required to separate rubber and 
fabric at a definite rate. 

The first method is much employed in testing mechanical goods. In 
testing belting, for example, a test piece is cut one inch wide and about 
six inches long. Two plies of fabric are separated; the end of one ply is 
mounted in a rigid position, while to the other ply is attached a weighted 
clamp. A mark is made where the test is to start, and the weighted clamp 
is then released. After a fixed time (generally 10 minutes), the amount 
of separation is measured. This test merely gives a minimum value, and 
does not measure the true adhesion. 

In the second method, one ply is fastened to the upper clamp of a test- 
ing machine, and the other ply to the lower one. The clamps are now 
separated at a uniform rate, usually 2" per minute, and an auto- 
graphic record made of the pull required to separate the two plies. This 
method not only shows the maximum strength of the adhesion, but gives 
the variation over the area tested, thus revealing any lack of uniformity. 

Ordinarily, the adhesion between two rubber plies cannot be tested in 
this manner, since the joint is usually stronger than either of the two 
compounds. However, the second method is available for testing adhesions 
such as are found in the acid cured splice of an inner tube, or between a 
hard rubber and a soft rubber compound. 

Heat Aging Tests. All of the tests described above are performed on 
test pieces in the condition as they come from the vulcanizers. Such tests 
give little, if any, idea of what the physical properties of the compound 
will be at some future time. Artificial means for aging have been sug- 
gested, but probably the most widely used is the one developed by W. C. 
Geer. 10 The test pieces are maintained at a constant temperature of 70C, 
and tests made at regular intervals until deterioration sets in. The most 
serious fault with this method is that the heating takes place in air, so 
that in addition to the deterioration caused by heating, we have the effect 
of the oxygen of the air. In a great many cases, such as tire frictions, 
breaker stocks, cushion stocks, etc., heat is undoubtedly the principal 
agency in deterioration. In such cases, the heating should be done in an 

9 Cf. Bureau of Standards Circular 38, Fourth Edition, pages 57-8, for a 
description of a convenient form of apparatus for making these tests. 

10 W. C. Geer, India Rubber World 55. 127-30 (1916) : W. C. Gecr and W. W. 
Evans, India Rubber World 6h 887-92 (1921). 



148 THE ANALYSIS OF RUBBER 

atmosphere of gas free from oxygen, or any gas which may have a tendency 
to react with rubber. 

The most which can be claimed for this test is that it is useful in com- 
paring compounds of about the same type, or of cures of the same com- 
pound. With oxygen excluded, it might be extended to include compounds 
of different types, which are likely to be exposed to the same degree of 
heat. Beyond this, we have not succeeded in developing anj'thing reliable 
in the way of accelerated aging. 



APPENDIX d. 

Table of Specific Gravities. 

Minimum Maximum 

Acetone 0.797 

Aluminium silicate 2.61 3.02 

Aluminum flake 2.56 2.65 

Ammonium carbonate 1.50 1.60 

Aniline 1.00 1.03 

Antimony, red 2.87 

golden 2.57 2.90 

" crimson 3.11 4.20 

" black 4.80 

Arsenic yellow 2.75 

Asbestine 2.60 2.82 

Asphalt 0.99 

liquid 0.99 

Trinidad 1.20 

Balata 1.05 

Barytes (blanc fixe) 4.20 4.92 

Beeswax 0.97 

Benzene 0.745 

Bitumen 1.07 1.16 

Black substitute 1.10 

Bone black 2.20 2.32 

Brown substitute 1.07 1.32 

Burgundy pitch 1.10 

Camphene 0.865 

Candelilla wax 0.99 

Carbon bisulfide 1.26 1.29 

Carbon black 1.68 1.89 

Carbon tetrachloride 1.61 

Carnauba wax 0.995 

Castor oil 0.958 

Ceresin 0.918 0.922 

Chloroform 1.52 

Chrome yellow, light 6.41 

" " medium 5.73 5.84 

" " deep 5.91 6.08 

Chrome green 524 5.44 

Clay, China 2.60 

Blue Ridge 2.55 

" Dixie 2.58 

Coal tar 1.05 1.27 

Cork 0.24 1.00 

Cork dust 1.16 

Corn oil 0.926 0.930 

Cotton 1.45 1.55 

Cottonseed oil 0.922 0.93 

Dimethylaniline 0.958 

149 



150 THE ANALYSIS OF RUBBER 

Minimum Maximum 

Dipheii3'lamine 1.16 

Fossil flour 2.00 2.60 

Fuller's earth 1.80 2.70 

Gasoline, 72-75 Be 0.700 0.707 

Glass, powdered 2.49 

Glue 1.30 

Glycerin 1.25 1.30 

Graphite 1.95 2.40 

Guttapercha 0.96 1.00 

Hexamethylenetetramine 1.25 

Indian red 4.80 525 

Infusorial earth 1.66 1.95 

Kaolin 2.75 

Lampblack 1.53 1.75 

Lead, chromate 5.65 6.12 

oleate 1.50 

" red 8.17 

" sublimed blue 6.40 

" " white 6.20 6.30 

white 6.10 6.75 

Leather fiber 1.40 

Lime 2.21 2.40 

Linseed oil 0.94 

Litharge 8.90 9.52 

Lithopone 3.60 4.25 

Magnesia 2.16 3.65 

Magnesium carbonate, light 1.74 2.22 

heavy 3.00 3.07 

oxide 3.20 

Mica 2.80 3.20 

Mineral rubber 1.00 1.06 

Montan wax 1.04 

Ochre 3.50 

Ozocerite 0.90 0.95 

Palm oil 0.94 

Paraffin 0.869 0.91 

oil 0.90 

wax 0.91 

Petrolatum 0.90 

Pine tar 1.05 

Pitch 1.23 1.28 

Prussian blue 196 

Red oxide 4.82 5.16 

Rosin 1.05 1.08 

Rosin oil 0.98 1.10 

Rubber, Accra flake 1.02 

" Amber crepe 0.92 

Assam 0.967 

Benguella 0.928 

Borneo 0.916 

" Cameroon 0.929 

" Cameta 0.916 

Caucho ball 0.915 

Centrals 0.93 

Congo 0.93 

Guayule 0.975 

extracted 0.995 

" Madagascar 0.915 



APPENDIX 151 

Minimum Maximum 

Rubber, Manicoba 0.93 

" Mozambique 0.939 

Niger flake 0.93 

" Para, coarse 0.95 

" fine 0.94 

Penang 0.918 

Pontianak 0.99 

" Roll brown crepe 0.95 

Senegal 0.929 

Sernamby 0.918 

Sierra Leone 0.923 

Singapore 0.937 

Smoked sheets 0.91 0.95 

" West Indies 0.935 

Starch 1.50 

Sulfur 1.96 2.07 

Sulfur chloride 1.69 1.17 

Talc 2.00 2.78 

Thiocarbanilide 1.30 

Tripoli 1.95 2.25 

Ultramarine 2.30 2.40 

Vaseline 0.84 0.945 

Venetian red 1.96 2.07 

Vermilion 7.89 8.10 

Wax tailings 1.00 1.08 

White substitute 1.04 1.14 

Whiting 2.60 2.72 

Wood pulp 1.43 1.46 

Yellow ochre 3.50 5.00 

Zinc, carbonate 4.42 4.45 

" oxide 5.38 5.60 

leaded 5.64 

" sulfate 3.62 

" sulfide 3.50 



SUBJECT INDEX. 



Note. Figures in Roman refer to pages ; figures in Italic refer 
to the article listed in the bibliography. 



Accelerators, inorganic, 42, 106, 200, 
4<)6, 408. 

Accelerators, organic, 38, 94, 96, 15, 
16, 41, 42, 43, 60, 78, 112, ISO, 
133, 138, 140, 141, 142, m, W, 
149, 150, 155, 195, 255, 256, 271, 
286, 287, 288, 289, 305, 323, 347, 
373, 386, 387, 403, 406, 4O8, 409, 
429, 430, 433, 434, 485, 462, 491, 
492, 493. 

Accelerators, ultra-rapid, 41, 59. 

Aging tests, 146, 147, 17, 59, 188, 
189, 357, 412, 456, 457. 

Aldehyde ammonia, 40. 

Aldehyde aniline, 39. 

Aluminium, 119. 

Aluminum flake, 45. 

Amber crepe, 20. 

Ammonium carbonate, 46. 

Analysis, Interpretation of, 4%1- 

Aniline (aniline oil), 38, 95, 358. 

Antimony, Crimson, 47, ^7. 

Antimony, Determination of, 102, 
119, 10, 18, 75, 76, 172, 173, 174, 
254, 299, 345, 355, 356, 365, 368, 
371, 481, 460. 

Antimony, Golden, 49, 7, 377, 378, 
443, 482, 494, 495. 

Asbestine, 46. 

Ash determinations, 83. 

Balloon fabrics, 24, 27, 28, 184, 154, 

836, 419. 
Barium carbonate, 105, 418. 
Barium, Determination of, 104 119 

418. 
Barytes (barium sulfate), 51. 
Bitumens (see mineral rubber). 
Blanc fixe (see barytes). 
Bolting, 140. 

Breaking-down rubber, 140. 
Brown pigments, 47. 
Burgundy pitch, 34. 

Calcium carbonate (see whiting). 
Calcium sulfate, 47, 49, 119, 494. 



Calendaring, 141. 

Carbonates, Determination of, 101, 

264. 
Carbon black (see gas black). 
Castilloa, 17. 
Caucho, 17. 
Cellulose, Determination of, 158 

219. 
Cements, 66, 142. 
Centrals, 17. 
Ceresin, 32, 55. 
Chinese blue, 47. 
Chromates, 108. 
Coagulation, 16. 
Coefficient of vulcanization, 25 

401, 454, 455. 
Compounding, 141. 
Compounds, Preparation of rubber, 

26 
Copper, 43, 99, 171, 181. 
Cottonseed oil, 34. 
Cure, Definition of, 14. 
Cure tests, Formulas for, 25. 
Curing tests, 24. 

Dimethylaniline, 39. 
Diphenylamine, 39, 96. 
Diphenylcarboimide, 40. 
Diphenylguanidine, 40. 
Dithiocarbamates, 41. 
Drying, 139. 
Dyes, 47, 88, 111. 

Elongation, 146. 
Ethylidene aniline, 39. 
Extraction apparatus, 68, 38, 260. 
Extract, Acetone, 68, 73, 156, 251. 
Extract, Alcoholic potash, 71, 844- 
Extract, Chloroform, 69, 71. 

Fillers, 37, 45, 112, 119, J, IS, 11 4, 

221, 259, 278, 308, 309, 326. 
Formulas, Calculation of, 117. 
Fossil flour, 48. 
Frictions, 147. 
Furfuramide, 40. 



153 



154 



SUBJECT INDEX 



Gas black, 48, 106, 137, 292, 380, 

381. 
Glue, 36, 108. 
Graphite, 50. 
Greens, 50. 
Guayule, 18, 170. 

Hevea Braziliensis, 16. 
Hexamethylenetetramine, 39, 95. 

Indian red, 51. 
Iron oxides, 51, 119. 

Landolphia, 17. 

Lead, Determination of, 18, 115, 

290, 359. 
Lead oleate, 44. 
Lead, Red, 43, 107. 
Lead, Sublimed blue, 44. 
Lead, Sublimed white, 44, 359. 
Lead, White, 43. 
Liebermann-Storch test, 35. 
Light, Action of, 337, 31,6, 420, 47 J, . 
Lime, 51. 

Litharge, 42, 13, 106, 374, 402, 475. 
Lithopone, 51, 120. 

Magnesium carbonate, 45, 119, 21, 

105, 301. 
Magnesium oxide (heavy magne- 
sia), 45, 119, 104, 301, 408, 480. 
Manganese, 58, 59. 
Master batches, 140, 141. 
Microanalysis, 114, 7.9, 98, 101, 198, 

199, 452, 496, 497, 498, 499, 500, 

501. 
Microphotographs, 114. 
Mineral analysis, 97. 
Mineral fillers, 29, 34, 56, 77, 177, 

220, 266, 278, 281, 339, 342, 41',, 

442. 
Mineral hydrocarbons, 31. 
Mineral oils, 34, 70. 
Mineral rubber, 30, 31, 70, 71, 39, 

83, 84, 145, 295, 310, 338. 
Mixing, 141. 
Moisture, 24, .$87. 

Nitrogen in crude rubber, Determi- 
nation of, 24, 364. 

Oil substitutes, 28, 73, 9, 12, 19, 95, 
127, 129, 146, 253, 340, 436, 448. 
Oil substitutes, Tests for, 29. 
Oils, Softening, 33, 82. 
Oils, Tar, 36, 80. 
Organic fillers, 37, 110, 118. 
Oxides, Determination of, 99. 
Ozokerite, 32. 



Palm Oil, 33. 
Pale crepe, 20. 
Paraffin, 32, 70. 

Paranitrosodimethylaniline, 40, 94. 
Petrolatum, 34. 
Piperidine, 41. 
Piperine, 41. 
Pontianak, 18, 87. 

Resins, Rubber, 21, 24, 52, 93, 122, 
234, 257, 277, 391, 402, 438, 446, 
477, 478, 490. 

Roll brown crepe, 20. 

Rosin, 35, 70. 

Rubber, Amber crepe, 20. 

Rubber, Analysis of, 11, 19, 31, 32, 
33, 44, 49, 54, 69, 70, 73, 96, 97, 
102, 116, 117, 119, 120, 123, 124, 
131, 1S5, 136, V,3, 160, 163, 166, 
175, 178, 179, 197, 207, 208, 211, 
214, 216, 217, 224, 237, 239, 262, 
270, 282, 298, 306, 311, 330, 332, 
343, 375, 382, 411, 413, 426, 439, 
463, 465, 467, 470, 472, 473. 

Rubber, Composition of crude, 22. 

Rubber compound, 13, 139. 

Rubber, Crude, 13, 21, 26, 37, 128, 
184, 185, 218, 422. 

Rubber, Definition of, 13. 

Rubber, Determination of, 70, IIS, 
48, 100, 190, 196, 240, 2'i>, 250, 
304, 471. 

Rubber, Difference methods for 
determining, 82. 

Rubber, Direct determination of 
(see Nitrosite, Wesson's, and te- 
trabromide methods), 3, 25, 26, 
45 21,! h 245, 333, 334, 415, 445. 

Rubber, Evaluation of, 8, 89, 215, 
360, 372. 

Rubber, Formula for, 12. 

Rubber hydrocarbons, 13. 

Rubber, Indirect method for deter- 
mining, 81, 350. 

Rubber, Insoluble matter in crude, 
21, 15, 35, 36, 180, 238, 354, 366, 
369, 383, 384, 393, 416. 

Rubber, Nitrosite method for de- 
termining, 79, 4, 5, 165, 193, 194, 
202, 204, 205, 206. 

Rubber, Pale crepe, 20. 

Rubber, Para, 16, 17, 227. 

Rubber, Plantation, 19. 

Rubber for analysis, Preparation of, 
23, 30, 296, 476, 486. 

Rubber, Reclaimed, 27, 66, 112, 6. 

Rubber, Roll brown crepe, 20. 

Rubber, Smoked crepe, 20. 

Rubber, Smoked sheets, 20. 



SUBJECT INDEX 



155 



Rubber substitutes, 28, 272. 

Rubber, Synthetic, 279, 308, 328, 
481. 

Rubber, Tetrabromide method for 
determining, 76, J,0, 50, 61, 62, 
68, 6k, 65, 67, 71, W , 161, 164, 
167 169 209, 220, 223, 22S, 229, 
232, 235, 2^8, 246, 248, 273, 283, 
294, 367, 370, 389, 390, 892, 441, 
441, 449. 

Rubber, Wesson's method for de- 
termining, 79, 80, 92, 424, 4%5, 
483, 484, 485. 

Sampling, 23, 64, /,17. 

Set, Permanent, 147. 

Silicates, 102. 

Sodium bicarbonate, 52. 

Sodium hydroxide, 42. 

Solvents for rubber, 81, 85, 188, 

263, 479. 
Specific gravity, HI, 103, 363. 
Specific gravity table, 149. 
Sponge rubber, 46, 110. 
Spreading, 142. 
Stress-strain curves, 146, 210, 861, 

376. 
Sulfates, 101, 1, 399. 
Sulfides, 100, 7, 132, 264, 899. 
Sulfites, 101. 

Sulfur, 37, 50, 118, 233, 812, 335. 
Sulfur, Combined (see sulfur of 

vulcanization). 
Sulfur, Free, 29, 88, 89, 29, 92, 152, 

268, 269, 330, 44O. 
Sulfur, Total, 84, 1, 2, 22, 52, 91, 

94, 99, 110, 152, 159, 162, 176, 

186, 187, 220, 236, 247, 252, 267, 

280, 293, 307, 327, 331, 348, 895, 

398, 407, 423, 439, 444, 458, 459, 

461, 469, 
Sulfur in fillers, 83, 93. 
Sulfur of vulcanization, 25, 91, 66, 

7//, 78, 153, 241. 
Sulfur chloride, 37, 60 90, 182, 



Talc, 52. 

Tensile product, 145. 

Tensile strength. 144. 

Tensile tests, 143, 258, 291, 309, 

352, 410, 458. 
Tensile tests, Machine for making, 

143. 
Tensile tests. Test pieces for, 144. 
Thiocarbanilide, 39, 95. 
Thiurams, 41. 
Triphenylguanidine, 40. 
Tubing, 142. 

Ultramarine, 52. 
Unsaponifiable matter, 73. 

Venice turpentine, 35. 
Vermilion, 53, 178, 174, 356. 
Vulcanization by selenium, 51. 
Vulcanization by ultra-violet rays, 

Pi 5 * 213 
Vulcanization, Cold, 60, 61, 46, 220. 
Vulcanization, Definition of, 1), 57. 
Vulcanization, Hot, 107, 108, 121, 

220, 284, 285, 374, 379, 397, 475. 
Vulcanization, Ostromuislenskii's 

theory of, 62, 68, 313, 314, 315, 

316, 817, 318, 819, 320, 321, 400. 
Vulcanization, Theory of, 57, 72, 

125, 126, 139, 225, 230, 281, 233, 

394, 404, 405, 428. _ 430, 4H, 466. 
Vulcanization with mixed gases, 20, 

825. 

Washing, 22, 139. 
Washing, Loss on, 22. 
Waxes, 33, 82. 
Waxy hydrocarbons, 74. 
Whiting, 53, 119. 

Yellow ochre, 53. 

Zinc, Determination of, 18, 115, 

290. 
Zinc oxide, 54, 120. 



